I've done a bunch of Big-A in my time, of all varieties, but I like shrooms the best. Less likely to be laced or cut with anything. We used to get liquid and put it in our eyes with a dropper. Can't really recall how much we did at once ... details fuzzy. Most of my bad trips were on cut LSD, usually laced with Strichnine or something.

Nitrous is cool. But go outside and get some air when you start turning blue or plae from o2 starvation. This step is critical if you're working off a tank. I like nitrous for the wah-wah-wah effect.