hiya, welcome to the forum!
To the best of my knowledge and from experience I don't think there is a way to condition yourself against pressure point strikes.
I can't quite remember where following explanation came from, I'm sure it was a Dillman tape (I'll have to go back through them to confirm), nor whether the juices I'm about to talk about actually exist, but its a great way of explaining it and I can vouch for it during the pain. Perhaps one of our resident guru's could give a more technical explanation. Anyway here goes:
When you hit a point it releases a kind of protective juice to ease the effect of the strike. If the point is struck again it will release about 60% of this juice, thus reducing the protection from the strike. On the third strike the body only releases about 30% of this juice to ease the effects of the strike. Basically this protective juice effectively runs out, so with each strike its effects multiply due to decrease in this protective juice.
A good way to prove this is to pick on a specific point, I usually use Large Intestine 10 (LI10) in the arm to prove this one. You can see the location of LI10
here! If you can find a willing partner, whap them with 3 low powered strikes (just hard enough to feel it), using the same ammount of power each time you do it. Pause for a couple of seconds between each one so your partner can tell what they feel like. I think you'll find that by the third strike they'll be asking you to stop!
Obviously only do this when your with a qualified instructor yada yada!
This three hit rule is sometimes used as a sneaky tactic at PP demonstrations to score an easy K.O. The demonstrator will show the points being used a couple of times before actually doing the real one, with each demo run they'll whap the points a little to soften up the recipient for the real strike.
I'll do some looking around to try and find out a more technical explanation rather than just "protective juices", and post it up if you're interested. That's unless someone else knows what the hell I'm talking about!