1. Don't Build from Scratch
Don't forget all the odds, ends, and knickknacks that can be turned into fantastic prop pieces or decorative accents for your outfit. Old broom handles make great bases for staffs and other weapons. Also, don't be afraid (with permission of course) to raid the attic and your parent's closets too. Never know what gems you'll find.
Also, garage sales for the win! One person's trash is another person's treasure. Looking for leather? You can often find old suitcases and rip the leather off...or salvage an old piece of leather furniture that's still in acceptable condition. It's a genius way to get real leather for next to nothing.
2.Build it Yourself
3. Plan Ahead
This not only allows you to keep your eyes open when hitting up the thrift store or garage sales, but it also allows you to buy items out of season -when they tend to be cheaper. Certain fabrics and colors do tend to come and go with the seasons. If you know what you'll need for a future cosplay can you purchase them when they're being clearanced off to make room for the next season's items. Additionally, having extra time to work with allows you to wait for that 40% coupon code to come back around.
Finally -the week after Halloween is the perfect time to pick up lots of costume goodies for cheap!
4. Build a Base Wardrobe
5. Know When to Splurge
Wigs are another example if you plan on using them more than once or twice. You generally want to go with quality to ensure they fit well, don't shed, can be styled, and easily maintained between cons or different outfits. Now these are only two examples, but you get the idea/point, right?
6. Odds & Ends
2. Don't underestimate the power of hot glue, E6000 glue, cardboard, paper mache, and duct tape.
3. Build costumes that will use up scrap materials from prior outfits. Also, buy in bulk to save on costs, especially when it comes to shipping expenses (worbla anyone??).
4. To help with patterning use newspaper, paper shopping bags, or dollar store tissue paper. Make mockups using cheap muslin or old bed sheets before using your more expensive fabric.
5. Packrat (within reason of course). If you think it could become part of a future cosplay costume, don't be afraid to save it. One great example is the extra buttons that tend to come with clothes nowadays.