While I'm not disagreeing with SexualChoc; I would like to point out one small thing. The "trigger" does not have to be an obscure or made-up word. It just has to be specific to the Dom or Domme. In other words, the Dom or Domme, must make it clear that it is ONLY to work when He or She says it. The word could be "the", but only work when it's said by your Top, if you have one.
Now, if you're looking for it to work outside of a Dom or Domme's control, then I would go with the above suggestion and make it very obscure or a fictitious word. You wouldn't want that to just happen unexpectedly at an inappropriate time, like, say, a funeral or something.
There are ways to condition sexual responses. It's not unheard of. A thought to give you a little extra boost in that direction would be to abstain from any touching yourself or contact with others at all for quite some time before beginning your quest. In other words, the hornier you are when you start, the more likely that you would be to get very hard, very quickly. That's what you're looking for to begin with. Associate that with a trigger as described above. A sound, a sight, a touch in a certain (not common) place could all work.
If you have a Dom or a Domme or at least someone who would be willing to play that role during your training/conditioning; it would definitely help. It could be done solo, but having someone pushing your buttons would be a greater help. As I said, having your desire at its peak and wanting so badly to be touched, will facilitate this greatly.
The trigger should come just before or exactly in time with what causes the reaction. Rinse, repeat as often as you're able. Then try the trigger alone without the physical encouragement and it should work. Depending on you as a person, this may take some time, or it may work the very first time. Positive reinforcement can help to "make it stick", so to speak. In other words, if you have a Dom or a Domme saying, "good boy", or other such praise or reward, then your mind will associate the condition response with what you and/or your Dom/Domme wants. I would suggest that the reward not be an orgasm, as it might make you lose your erection, which could disassociate the trigger. Work on getting hard on command first and making it a part of you, and worry about orgasms later.
Also, when working on the conditioning, you may be tempted to speed up the process of going limp again to get to the point where you can try again to get hard on command. Be careful with this. The best way is to relax and take your mind off of anything that would stimulate you. Things to avoid would be cold showers, thinking of something icky, etc. Don't associate negative things like that with this process. Better to be patient than to get yourself trained to get hard on command, only to find out that the thing that makes you go became part of it by accident and find yourself losing the erection immediately after.
Above all else, you have to be willing to surrender that control over to the command and/or the Top. You have to be willing to let it happen whenever it occurs and while you're in the training process. In reality, that's how it works for something like this. You're a thinking, analyzing person, and you know what is going on. If you really don't want it to happen, it won't. However, as it seems from your post, you really would like it, probably a lot, so just relax and let it happen, it'll be fun for you. Just make sure you trust anyone to whom you give the keys.
I hope this helps. Good luck to you.
