2. Solid colored t-shirts and/or sweats- They can be the foundation for so many costumes! I bought solid red, long-sleeved shirts for my husband and I the year we went as the Incredibles. I printed out the logo on regular paper and taped it to the shirts. It looked great!
3. The remnant bin at Joann's- Around Halloween, the fabric cutting line at Joann's is crazy! But after a big weekend (big coupons, holiday, whatever) the fabric remnant bins are full of fun stuff. You might even find exactly the kind of fabric you wanted (or never knew you wanted till you saw it). And best of all, all the remnants are 50% off!
4. Felt- Available in paper sized pieces in a rainbow of colors at a multitude of stores, felt is some handy stuff. If you want something a little nicer than paper stuck to your solid color shirt, felt is incredibly easy to work with and has a nice look. You can sew it, glue it, or even tape it on. On my husband's Mike costume, I used felt to make the eye and the mouth. I taped it on because I was lazy. It held up alright through the night, but if we had wanted to get multiple wearings out of it, a more permanent adhesive would have been better
5. Group costumes- Having a group costume is fun. Sometimes it can be tricky (like when your kids start developing opinions on their Halloween costumes) but having a group theme can make it more obvious to other people what you are supposed to be. When we did Monsters INC, people who just saw our daughter (she was Boo in her pink shirt and purple pants) didn't get it. But when 'Mike' was holding her hand, it became pretty obvious.