I've been making lunch money for festival and getting ferocious headaches this way for about sixteen years now. I also spent a few years altering ready-to-wear for a local tailoring shop. Here are a few things I've learned...

• NEVER, EVER, EVER make a promise you can't keep. It hurts your credibility, and that's the hardest thing to recover. It is better to turn someone down. There is so much bad work out there, and people requesting sewing many times already feel that they're at the mercy of the person doing the work. Don't do that to your customers.
• Do what you say you'll do then do a little more. A real easy way to do this is don't tell the customer everything you're going to do, so that little things can make a bigger impact. Making a shirt with a pointed collar? Put stays in to keep the points from curling. Cost: $1.29 and fifteen minutes. Return: more business.
• You aren't going to make any money. I realize this sounds defeatist, but you aren't. As a computer consultant (my day job) I bill between $50.00 and $175/hour. Compare this to costuming: usually between $3.00 and 5.00/hour if I'm lucky. There hasn't been a ren fest costume yet where I made anywhere near what my time is worth on custom work. I do it to pay for my faire habit...I don't expect to get rich. Still want to try? I have some advice on that subject, too.
• Fittings Even if you don't need them, your clients do. It makes them feel like there is progress being made, and a competently performed fitting shows them you know what you're doing. (And, to be honest, you do need them. Nobody's that good.)
• If you make another costume for someone from a pattern you used before, take their measurements again if it's been more than a month. A person can lose half an inch just between breakfast and supper...think what could happen over a month or more! (It helps if you date the patterns and the measurement sheet you drafted them from)
• Attach worth to your work if you don't charge enough, no one will think you're worth doing business with. Be careful about charging too much, though, too. Look at what others around you are charging, figure what your time is worth, compare the two . The ren faire market typically won't support really high prices.
• Don't work for free especially if the person asking for it is a friend, and don't ask anyone to either. Barter if you like, trade favors, whatever. Keep business and friendship separate. On those occasions when I do make something without accepting payment, I keep the costume (any costumes in your shop that you aren't wearing right this minute? Make them available for rent!).
• Getting more work than you can handle? Raise your prices. When the pace slows down, lower them...a little. Keep adjusting until you have as much work as you want. (Note: it may take years to find the balance.)

Getting Work Done

• Put your stuff in its own room in the house. Have a 'shop' where you can "leave yer kit lyin' around". Put a lock on the door. Makes a great place to wrap presents around the holidays, too.
• Take on an apprentice. My costuming professor taught this way. Oh, sure , you had to take all the requisite classes, but if you really wanted to learn, you became an apprentice. Apprentices do all the grunt work, like making bias tape and piping, and cutting out garment pieces, and while it sounds menial it's a great place to learn the basics. In an article she wrote for Threads© Magazine Anne Stewart, kiltmaker in upstate New York, said she had to make 1000 hand-worked buttonholes in scrap fabric before her grandfather would even let her touch a piece of tartan! Compared to that, we got off light.