This is a really embaressing situation for you to be in, I know, I've been there myself. Now, the key to being a successful magician is PRACTICE. There's no point in rushing into a trick when you haven't learn't how to master it correctly. I practice my tricks about 20-30 times before I show it to somebody, but don't show it to someone just because your in a rush to show them, take your time and be patient. Another way to helping you to master magic is to use the right props, such as good quality cards. I find that the best brand of cards are 'BICYCLE cards'(See Magic Merchandise). Some tricks can be perfected by doing them in front of a mirror, however this isn't a good way for some tricks as some require certain angles for your audience and you to be in. There are two way these tricks can be rehersed.
1. Film yourself doing the tricks with a video camera, then play it back, and see where you need to improve until you have perfected the trick.
2. Perform the trick to a fellow magician, and ask them to say where you need to improve.
3. I always show my tricks to my mum, I rarely slip up, but if I do, she tells me some I know what to do or what not do do next time.