Writing your own scripts is always the best thing to do, although if you're doing self hypnosis I find the best thing is to know where you want to go and follow your mind, for example if an image should appear interact with it or observe it or follow it.
Another thing is I find it helpful to use the same induction consistently, with the same music, and that can train your mind to access deeper and deeper states of trance on command.
If you want to write your own scripts for self hypnosis, and something that i've done that i've found to be somewhat effective is tape recording myself. Although I have to admit i often fall asleep, but hey, if it's mostly positive suggestion then sure bring it on!!
As for the best triggers, some typical ones for inductions are going down a staircase , or an elevator. Counting down (from 10-1, or 5-1). Otherwise it depends what you want to trigger. I find it really helpful to work with someone when setting up triggers, because then you can go as deep as you need and not have to think about what you're doing next. I feel like while doing self hypnosis there's always a part of me that has to remain at least semi-conscious... (((But I guess the true power of trance is when you can trance out and at will yet remain in an aware state, once again depending on what you want to do. I guess the best analogy is Lucid Dreaming.)))
Although self hypnosis can be ideal for giving yourself positive suggestions and working with metaphors and visualization.
Hypnotherapy is great, very powerful, in fact infinitely so!!

I hope that gives you something to work with...
As I said, there are so many variables, depending on what you want to do...

I make my own CDs for clients. Use scripts off the Net or from a book I bought, but modify them according to the client's needs.I use music with subliminal messages and slow heartbeat as background, and find that I am already somewhat anchored to that music, in that it gets me into a slowed down, somewhat tranced out state when I hear it.

I think its lot of work to completely write all of your own scripts, but modifying or combining other scripts works well for mee. Perhaps after many years experience I may completely write my own................ but not yet : )