A good script can elevate tricks into magic. Just as you would never perform an effect without knowing what you are going to do, you should not perform an effect without knowing what you are going to say. So here are my tips:
1. A good script goes beyond describing what you are doing with the props (i.e. say-do-see patter). Props aren't really interesting, you are.
2. As Rick said, make it relevant to your audience. Celebrities, video games, sports, etc.
3. Write it out, read it aloud, revise. Let it sit for a couple of days, read it out loud, revise again. Repeat as necessary.
4. Use the script to enhance the effect. What you say can provide misdirection, provide an out or cause a spectator to remember something which did or did not happen. It should enhance the effect, not just gomalong for the ride because you need something to say but also not to overwhelm the effect (e.g. a multiplying sponge ball routine about overpopulation and hunger ).
5. Make it a conversation. Close-up magic should be a conversation. Ask questions, listen to your spectators and react.
6. Use IF, THEN. IF the spectator says or does this, THEN I will say or do that. It makes your magic seem more personal or impromptu.
7. Make your script and the effect like a good story. It should have a beginning, a middle and an end. It should have humor, drama, emotion and/or meaning.
8. Have the spectators get "there" a moment before you do. "There" being the reveal. That is, they realize what SHOULD happen a moment before it does. That makes them WANT the magic to happen.
9. Rehearse the script until it becomes like a story you've told a hundred of times. You know, like the time when you were in college and...
10. Let the script reflect your excitement and love of magic (yeah, I've been reading lots of Tamariz lately).