If you don't know what it could be, how do you know that there is an actual post-hypnotic suggestion in place?
Anyway, post-hypnotic suggestions are not really all that different from many things that happen to people naturally, such as phobias. A phobia is pretty much a post-hypnotic suggestion you accidentally give yourself. As such, the procedure is pretty much the same (and enrich is right that there are tons of ways to do it).
It's often tricky (but far from impossible) to hunt these things down on your own... and sometimes it's tricky to get rid of them even once you have found them. After all, if it was easy to change conditioned responses, there wouldn't be so many phobics and folks with OCD and so on.
I think your best bet would be to contact a hypnotist (and preferably a good one) and arrange something. Quite a few of the frequent posters here are worth a recommendation.
I'd be happy to talk to you on Skype (or messengers or whatever) if you want to know more.