If you're on something relativly safe, like weed, lsd, mushrooms, anything that doesn't usually kill people, all you need to do is keep yourself occupied and reminds yourself that you are only under the influence of a drug and it will go away soon. Doing something productive during a trip can make it much better, going for a walk with a bunch of friends in the forest on LSD will probably make your trip a lot better than sitting alone in the dark.

And if all else fails remind yourself you just payed good money for that shit and if you're gonna die, you might as well get your moneys worth first.