Make sure to be around people that won't try to fuck with you, make sure you're in good spirits before indulging, make sure you remember that it will eventually end, and relax.
I've only done it once, it was a scary yet spiritual experience for me, I felt like there were no more rules, I felt like my consciousness was separate from my body, and it was hard to hold on to thoughts and ideas for very long (I did this with my best friend) before being distracted by the next thought. It's hard to stay focused because every thought kind of sends you down a pathway, swerving into the next thought without end.
If you start feeling bad, just remember that it WILL end. Oh, and my head felt swollen--gave me a headache into the next day.
Enjoy, if that is what you choose.