I've been performing magic for about two years now. Usually, I'll carry around a deck of cards or a couple of coins in my pocket to perform with. Lately, I've noticed that after almost every single effect I perform, I'm always asked the same question, "Can you do it again". I usually avoid repeating an effect. What do ya'll do?
Matthew

Tell them you never repeat a trick because then there's no surprise.

Be prepared.
Do tricks that lend themselves to repetition, like mind reading with an 8/King stack - this is even better when it's repeated.
If you present an ACR routine, pause after the card appears on top of the deck the first time and let them say "do it again" (they don't' know you were going to anyway!).
Coins across can be presented the same way, if you pause after the first migration, get the idea?
You can also just say, "sure" and then do something else. I hate saying no to a spectator, but I also like to keep things on my terms and keep the surprises coming.

Well DDecae, there are certain effects you can repeat like 'coins across' and the Ambitous card routine. Unfortunately, many of the effects I perform do not have several phases like the two routines you mentioned. I thought it would be easy enough to just say 'sorry, but no' and not perform an effect again when someone asks me to. The only problem is a lot of people will dare me and say something like 'I bet you couldn't do it again' or they'll get annoyed.
Matthew

You tell them first time it's magic the second time it's educational.

Matthew I feel your pain. You've got a tough crowd there - I think kids are probably the worst at this, they are always saying "do it again". But I guess I hear that from kids because I'm just doing quick little impromptu tricks for kids.

I'd like to help you. Can you give an example of tricks you are doing that are always asked to be repeated?

The best way to deal with this is to say "well if you liked that, check this one..." and do another routine.
If you don't want to do anything else just say something like howie3 suggested, just don't be rude while you are saying "No!".

Reject the audience with a joke like "Oh! My doctor says I can only do this once per day. "
They know what you are trying to say and probably stop asking you to do it after a short laughter.

Something along JoeBlack`s line works fine and be ready to do another trick, usually with the same props, if possible. Maybe as I sometimes explain: "that one was one of my older tricks, here`s a newer one", indicating that you want to show them the latest magic. In other words just keep the magic moving along. To end sometimes I let them know that I`m about to pause or finish by saying "and now for my last trick".

John

So go ahead and do it again!

I hear this a lot, and usually reply to "Can you do that again!" with "Absolutely!" and then a grin.

or

"Even better, I can show you something more incredible!"

Somtimes though it is inevitable that you have to kind of ignore the loud mouth who keeps saying. "Hey do that trick with the glass again" with a sideways look telegraph that you really can't repeat a trick. His wife or someone will explain it to him.

Joe & Tom have given the best advice here. As Tom said, say something like "if you liked that you'll love this" & move on to another effect. Some effects are made to be repeated so maybe have a couple of those in your arsenal.
Another tip is to walk away while their jaws are still gaping in disbelief from what you've just done.

Do a different version of the same effect.

I have made it a personal policy to walk away if someone asks "Do it again!" because it makes certain I end on a high note. So... I thank them and put my hands in my pockets.

Well said ToasterofDoom.

Sometimes it bears repeating that as the magician, you are the one in control. It's a hard lesson I've had to learn. It's my own preference to do one very strong effect I know I've mastered and then stop.

I usually say: "Sorry, but I've practiced these all together.." then go into the next one.

Sir Richard.

I say sure ok and then do something different

I usually use a phrase along the lines of "the magicians code prevents me from performing the same effect twice in a row, because the first time is entertaining, the second is educating" This is always said with a smile, never rudely. 99% of the people I have come across let it go at that. As you know though, there are some who are just never satisfied.

"sure I *can* do it again, but will I? Probably not, I got some better stuff."

"Well I can do that, or I could say **** you"
(jokingly though)