|Frank cape lining on dress form.|
If they have supplied you with materials or a deposit you are beholden to them. This reinforces my point about limiting the number of requests you accept - because it's not fair to take someone's materials and then sit on them because you were busy or over-extended. A lot of us have been there (myself included), and it's a huge mess to clean up. Honor your current obligations before making new ones. You just never know what obstacles may pop up in life, and it's better to wrap up a short list of obligations than a long one.
The next biggest pitfall? Don't get taken advantage of. Even if you're doing it out of the kindness of your heart keep track of your time and every expense you incur. If you're cursing the costume in the end because it was more than you bargained for chances are your own boundaries (or lack there-of) are what failed you. You can be successful if you are completely realistic.
If you charge for your work it can be tricky knowing how much to ask for. A general rule of thumb is 3x the cost of materials. So if you spent $20 on fabric and supplies, you should charge $60 for the finished costume. But if it requires additional skills - like hand-sewing hundreds of sequins on a corset - you should definitely factor that in your price. What's important here is that you feel it was worth it, otherwise why would you do it again? If you want a nicer sewing machine or a serger, you're going to have to count every dollar.
As for the people who will "pay you later" - you really need to be firm here. Nobody needs a Rocky costume that badly. If they can't pay the pre-arranged amount do not give them the costume under any circumstances. If you do, consider it a gift because the odds of recovering anything are slim to none. It's not because people aim to be shitty to you, but because Nobody needs a Rocky costume that badly - and the next time they have $20 in their hand you can be sure it's going towards something more important to them (their next meal, gas for their car, cigarettes, etc.) Paying you back will be at the bottom of their selfish list - and they're selfish because they asked you to front them for their responsibility in the first place. Don't diminish your boundaries because they did. And don't expect respect from other people if you don't respect yourself. Simply let them know "I'd be glad to hold it in a safe place for you."
Don't be hard on yourself when you make mistakes. It's not productive. Create a solution and follow it through. If you get upset and quit you've learned nothing and you will find yourself in the same place again and again.