I drink caffine all the time while high, doesn't kill it. I think its actually nice, cos if you take it early enough in the high, even with kinda shitty weed you don't get as burnt out at the end. Plus the weed smooths everything out so weed + caffine = can stay up forever... Though after a while... you loose the desire to haha.

Also nicotine def enhances my high, but I save it for special sessions, for my nightly smoke, I just stick to weed.