I'm thinking of handing out a pre-stamped self-addressed postcard after my show. The postcard would have questions like:
"What was your favorite part of the show?"
"What is you least favorite part?"
"Would you recommend this magician?"

What other questions should I ask?

"If you were to tell someone else about this performance, what would you say?"
-This can give you great quotes for a promo kit. Leave lots of room for them to write.

"Would you consider hiring this magician again?"
-Gives you an idea if they want you back. Keep in contact with them and send out promo information.

On a scale of 1 to 10, how did.....
-your guests like the magic?
-you feel about the price for the performance given?
-etc.

Using a scale (I use 1-5) is better since people have a difficult time responding to writing out answers to questions. In mine I ask about the performer - dress, interaction with kids, the show - tricks, appearance, pricing, value and things like that.

I also use the question, how would you recommend me to a friend or co-worker. This does indeed get lots of great quotes.

I get a 95%+ return rate so I must be making it easy to respond. It is printed on 8 1/2 x 11 and comes with SASE.

I book through an agency and they send out a sheet with every contract. These are the questions.

1. Was the artist prompt and professional?
2. Was the artist courteous and friendly?
3. Appearence of artist.
4. Quality of show.
5. Response of show.
6. Value of performance.
7. Your overall satisfaction.
8. Would you like to schedule another program?
9. Would you recommend this program to others? If yes, who?
10. Personal comments:

You probably won't use all of these but you might get some ideas.

Thanks! Any more?

Basically, I'm looking for quotes to use in my promo material.

Use the quotes, but also be interested in the other information you can gather from a good questionaire. Every little bit does help you to learn about the show you perform and if there needs to be any tuning or fixing in the show. Not always does the audience see the show as you see it yourself.

I also like the grading system. Keep your questions limited and make it easy for your audience to respond back to you. If it is easy for them, you will get a better return rate.

I also always include my questionare with a nice thank you note. The client will appreciate the extra effort you put in and may want to answer your questions more because of your generousity.

Should I send it in the mail or hand it to them after the show?

I would send it in the mail with a thank you note. This way they are more apt to read it and will then put your name back in their minds. If given right after a performance, chances are so much is going on, that they may misplace it. I would mail it with a thank you letter a day or two after the gig.

Check this out:

What do you guys think?

http://www.SalazarMagic.com/Reviews.htm

If you want feedback so you can improve your show then asking openended questions is not the best way to go. I have a degree in Sociology and statistics and the most important part of any study is to make your results measurable. I.e. you need to be able to make pretty pie charts with them.

If you are looking for quotes to use in publicity then ask questions that will get good quotes. If you ask "Why was your favourite part?" you'll get answer which describe your show without being positive.

e.g. "I liked the bit where the magician made the rabbit appear" - which isn't a great quote

Ask questions about WHY they enjoyed the show as opposed to WHAT they enjoyed to ensure you get emotional responses. "Why did you like or dislike the show?"

Makes sense.