|Some of the most effective costumes are simple ones that merely suggest how others should see you. These have the added benefit of being easier, quicker, and cheaper to put together, not to mention more comfortable and convenient to wear for an entire weekend while playing a game.
Try finding appropriate clothing in your closet or in a local secondhand or bargain-basement store. Characters who live and work in space, as well as military figures or others who spend a lot of time in ships, probably wear jumpsuits similar to those you see at gas stations, maybe with some creative pins, fancy buttons, or patches (try doing your own with magic marker on scrap cloth) to denote rank and affiliation. Military figures might also find fancy coats or tunics which look like formal uniforms. Businesscreatures and diplomats would likely wear something that is easily suggested with an ordinary business suit. Alternatively, they might have some form of ceremonial robes that a bathrobe or kaftan could stand in for. Scientists might wear the traditional white lab coat. If you're Kikkita, consider a kimono jacket with bright colors and interesting patterns. And of course, a well-chosen hat can be very effective for any costume. Use your imagination as you paw through the racks, and look for things you can chop up and modify if they're not quite what you want.
Since Hallowe'en is just past, you may be able to pick up interesting makeup and accessories on the cheap (try fabric and craft stores as well as costume and general merchandise places). If you're Kikkita, consider a pair of deely-boppers -- you can always put something tamer than a pumpkin on the ends. A sparkly or geodesic plastic ball can be cut in half and put on like earmuffs, to make cool bug-eyes. Deogen, Igratana, and Zavirene may be able to find cheap capes to serve as wings. Face paint is generally easy to find -- don't forget to look for ridiculous lipstick colors at the drugstore, such as silver for Aisi or other colors for people with hair, feathers, or scales on their faces or hands. Tokaji Aszu and aborigines may be able to find good nature-derived decorations, beads, leather, and the like at craft stores. If you're the violent type, you can pick up a blaster or ceremonial sword at any toy store (but nothing too realistic, please -- the GMs will not allow anything questionable). Winky-blinky light and noise toys, as well as erector sets or other techy toys, may be good sources for Aisi. Try just walking around and looking at things; see if anything strikes your fancy.
If you're determined to construct your own costume, good luck. Remember that it doesn't have to last beyond the weekend, so you don't have to get too elaborate or be too careful in the construction. Try the remnant bins in fabric stores -- small pieces for reduced prices. Also take a good look at some of the crazy specialty fabrics and notions; you can do very creative things with small quantities, especially if you're trying to simulate scales, feathers, or fur. Think about everything you'll need in the construction (thread, zippers, buttons, elastic, etc) so you don't have to make too many trips to the store.
Tunics, dresses, robes, or other similar garments:
Take a string or tape measure, start it from where you want the garment to end (presumably somewhere between your waist and ankles), run it up over your shoulder, and back down to where it started. That's how long a rectangle of fabric you'll want. The width is more open to your judgement, but should probably be at least a few inches more than the width of your shoulders or hips. If the tunic will go more than halfway down your thighs, you'll have to either leave it open (not a good idea if you're not going to wear pants underneath) or flare it wide enough to make a skirt you can walk in. Fold the rectangle in half crosswise (bring the short ends together) and cut a hole (or a hole and a front slit) at the fold large enough for you to put your head through. Pop the thing over your head, belt it at your waist, and hey, presto! you're done. If you want to not have little stringies shedding, you can hem the raw edges, or just seal them with a little Fray-Check (fabric glue). For a rather different look, try just belting the front half, leaving the back half to float behind you. You can sew up the sides if you want, leaving holes for your arms -- be sure to plan ahead and leave a seam allowance. You can cut the front open down the middle, making something that looks more like a coat. If you want sleeves, try to get fabric wide enough (60") that you can cut the tunic out in one huge piece, shaped like a cross. Fold the fabric in half lengthwise and draw what you want the silhouette to look like -- you'll be cutting up the sides, to the armpits, turning to parallel the fold, and heading down to your wrists (take a shirt and fold it in half down the front if you're not sure what this should look like). Make sure to make it roomy enough that you can sew it up and still move around -- if you must have a tight-fitting garment, you're probably better off with a stretchy knit. Remember, you can always take it in. Aisi might try a sandwich-board style cardboard construct, rather than the traditional but highly uncomfortable box with armholes.
Wings, capes, and the like, the easy way:
Decide how long you want it to be from the back of your neck to the bottom hem. You'll need fabric at least that wide (if you can't get fabric that wide, you'll need to make the cape in several pieces, and that's too complex to describe here) and twice that long. Fold the fabric in half crosswise (so you get a square) and use a meterstick or piece of string as a compass, drawing a quarter-circle with the center at one end of the folded side. Draw a smaller, concentric quarter-circle, to make a cutout for your neck. When you cut the piece out and unfold it, you will have a half-circle with a bite out of the middle of the straight edge. Finish the edges if you wish, as described above. You can fasten this around your neck with a pin (a safety pin hidden on the inside works fine), or if you don't like being strangled, try pinning the corners to your shoulders. Alternative: attach a piece of string to each corner, put the cape on over your shoulders, bring the strings under your armpits, and then tie them together behind your back.
Kikkita wing-cases or carapaces may be most easily constructed out of cardboard and attached to your shoulders any of the ways you would attach a cape. Make them backpack-sized rather than knee-length, or you may have trouble sitting down. Cover with construction paper and color in with markers.
Tails and other appendages:
The most important design criterion for a tail is to make sure you will be able to sit down comfortably. An easy way to attach a tail is to make it hook around your belt (you can hide the linkage under a shirttail). You can cut out a long rectangle of cloth, sew it into a tube (remember that it will look much narrower when stuffed!), turn it inside out and stuff it. Try unbending a coat hanger and threading it down inside before you stuff the tail, to make it stand away from your legs, or connect the end of the tail to one wrist with a long piece of fishline. If you want a broad, flat tail, you could construct it from cardboard, or you could make a framework from coat hangers and glue or sew paper or fabric over it. Extra limbs can be made in similar ways -- try attaching a laundry glove to an extra sleeve, loosely stuffing it, and pinning it to your side under a loose open-sided tunic or through a hole cut in the side of your garment. A coat hanger inside, or fishline between your real wrist and fake wrist will help make it look more real.
Remember that most of the time people are talking to you, they'll be looking at your face, so a little mask can go a long way. Try a half-mask (one that covers your eyes and nose), or a pair of ears, a nose, buggy mouth-parts between your eyes, or something else that doesn't cover your whole face -- those tend to be more comfortable. To attach something to the top or sides of your head, try putting it on a girl's plastic hairband. If you wear glasses, make sure you leave room for them, but realize that you can attach things to them, too. To construct a mask, try using oaktag or other lightweight cardboard, coloring it with magic markers. If you want to get fancy, you can cover your oaktag base with papier-mƒch‚. Mix flour and water together to get a thin, runny paste, and cook it over low heat until it is thick and translucent. Cut some junk mail into convenient strips, dip them in the paste, and lay them over the base, covering the inside as well as the outside. If you want to build up fancy structures, dip wadded-up tissues in the paste and mold it to shape. Three layers of papier-mƒch‚ is usually sufficient to give you a lightweight, flexible mask. Let it dry until it is no longer cool to the touch (this will take at least overnight, and may take several days, so start early). You can color on it with markers, paint it with acrylics, or use a 1:1 mixture of watercolor paint and white glue.
Hoods: Try disguising the sides and back of your head, an Aisi with a cardboard box, other species with a hood or cowl. Truly simple -- take a piece of fabric long enough to go from one shoulder to your neck, up around your head, and over to the other shoulder; and wide enough to go from your forehead, over your head, and down at least past your neck. Put it over your head and tie a piece of string loosely around your neck to hold it in place. More complex, but still easy -- trace the silhouette of a jacket hood onto two thicknesses of fabric, add seam allowances, and sew the two halves together down the back of your head. Pins and bobby pins should hold it in place. You can extend it down your back (and front) if you want, and even hook it to your belt for stability. Ears, spines, and the like can be added, and will stand up reasonably well as long as you keep their weight down.
Remember, you don't have to costume at all. But if you do, have fun doing it. Good luck, and don't panic!