Pressure points or pinching are your best bets. Pretty much everything else will require a lot more preparation and/or will risk accidental application. It really depends on how discrete you want to be.

The only real way to learn pressure points is to practice them. You should already have a half-way decent idea where your sensitive spots are, so poke and squeeze your own body. You can find pretty much all the ones you can apply discretely that way and you will have the benefit of knowing what they feel like and how effective they are.