It's all about timing for me. If my partner thrusts against my g-spot early in sex, I usually don't feel that great from it. After we've been going a little while, or if there was some fingering or toy play beforehand, then it feels much better when he hits it. I need time to warm up and turn on and be ready to receive stimulation there.
I find if I can look at my partner during sex and concentrate on him and how good he looks and sounds, then my mind's much less likely to wander. Also, if I talk to him, or get him to talk to me, that really keeps me engaged in the moment.