The more money you spend on a costume, the more important it is to get swatches before you buy your fabric. (The amount of money you spend is relative. If money's tight then anything can seem expensive.) In an effort to be a smarter shopper -- and to keep from adding yards and yards of spare fabric to my ever-decreasing storage space at home -- I went swatching this morning. It was a skill I learned in the costume shop in college, and I've improved on that technique over the years.
These are my swatches from today. I was shopping for two different projects. You can see I titled them "tail" for my mermaid tail and "robe" for my robe project. I didn't make notes about cleaning instructions because none of these things will be able to be washed at all. I knew that going into it.
Get the clerks in the fabric stores to cut the swatches for you. Sometimes they scrimp, giving you a sliver that will only give you a color sampling. It's important to be able to test the hand (feel) of the fabric, stretch, transparency, and texture. The shops in LA tend to give decent swatches. If they're stingy with the swatches or if you need a little more fabric to test the drape, buy the least amount you can form the vendor. I bought a yard of the robe fabric because I needed a larger sample and it was only $3 a yard. (I stuck the business card to the tape over a swatch I cut from my purchase so I'd have a reference when it was time to shop for real.)
This is the yard of fabric I bought. I needed enough to examine the drape and to see how it would look with the boas I have. In fact, I have enough to get naked and see how sheer it is on other parts of my body (besides my arm).
I need to start keeping a rolodex of swatches because I've gone back and purchased fabric I swatched for other projects. It would save me time when I go hunting for that one fabric a good six months after the original project is complete.