Costume satin is built for cheapo Halloween costumes which are meant to last one day.
Fabric choice is key when it comes to making my costumes. After color, drape, weight, texture and price are my next priorities. I stick to cotton twill and linen-look cotton/rayon/poly blend for 80% of what I do, with a mixture of moleskin/upholstry suede, suiting, and certain polyester wovens for the rest. Except in the case of specialty projects, like the velvet used for the outfits in my avatar (justified in the case of Renaissance style). I've got my first large-scale satin project coming up too, but one, it's matte bridal satin (16+ yards of the stuff, dear lord), and two, it's for a CLAMP design. Satin was the only fabric that made sense besides silk, and I wasn't going to pay for 16 yards of real silk, especially when what I could find was far too lightweight for what I needed.
Hair (wigs, to be more precise) and shoes are pet peeves of mine. I know a lot of people can't afford a super-nice wig, so I not to think about it too much. To me, the extra money's worth it because nicer wigs not only look better, they last longer. I've got a medium-quality wig that I've probably worn 20 times and owned for 4 years, and it still looks fantastic because the fiber's good (Sepia is a wonderful brand). If you're not that picky or don't plan on wearing it more than a couple of times that's different, but if not, why not buy something you won't have to replace by next con because the original's totally matted or fuzzy? It doesn't have to be really expensive. I've made a really nice wig by sewing two $16 costume wigs together (lovely fiber, actually), and I only went that far because I wanted it to be super-thick.
Don't wear dirty sneakers with your nice costume unless the character actually wears dirty sneakers. It doesn't have to be spot on accurate...shoes can be one of the hardest aspects of a costume...but try to go for the right family at least. Clean flip flops if you can't buy pricy geta sandals, brown or black slip ons for school shoes. Walmart, Payless, and thrift stores are full of cheap options if you don't have something that works in your closet. That's where I get most of my shoes.
And something everyone can do to make their cosplay look ten times better: Iron! Starch if you can. Even if it's a t-shirt, it'll look fresher and newer if it's clean and crisp.
