ledhendrix, a few things.
First off, on the "bad trip" thing ... it is possible for someone to have a "bad trip" that actually damages them, so to speak. However, this is also easy to avoid if one approaches the issue with any degree of responsibility and wisdom. A psychedelic experience can be traumatic (and therefore damaging) as a result of a few things, but barring any unexpected catastrophes happening while the trip is going on (basically anything that's really really scary or hurtful and would be regardless of any drug influence), the only way a person can damage themselves with psychedelics is by abusing them pretty flagrantly, and that's easy to avoid if you have even the slightest motivation to do so (that is to say, an inkling of respect for yourself).
I was going to go on with a few miscellaneous tips but I figure this is as good of a time as any to go all out and draw up a guide to first-time tripping.
Dosages for first times:
1 hit of LSD
1.7-2.4 grams (1/16th - 1/12th ounce) dried mushrooms
GROUND RULES:
1. Cars are REAL.
2. Fire BURNS.
3. You CANNOT fly.
4. Trust your friends. (This is of course only applicable in situations where your friends are trustworthy. If they are not, do not take drugs with them.)
Things to gravitate towards:
1. Your favorite music, art, media, etc. ... basically, anything that is an expression of beauty and/or creativity, and that you consider to be particularly pleasant/enjoyable/meaningful, is probably going to be a good thing to have around.
2. Outdoor green spaces, places with a lot of life. Nearby state parks are great (make sure you're familiar with the law enforcement situation and local bylaws, of course, don't want to be picked up by cops late at night for something as simple as trespassing). This also lowers the chance of being forced into uncomfortable interactions with strangers.
3. Familiar settings that have pleasant memories associated with them, or generally supportive overtones. It's good to feel safe and at home sometimes.
4. Relative calm and quiet is probably a plus, unless you get off on overstimulation. Having the capacity to focus yourself on something intently can be important.
5. Opportunities for free play and exploration in general are basically optimal for tripping, especially first-time tripping.
Things to avoid:
1. Strangers, and crowds, especially crowds of strangers. You don't want to have to deal with that.
2. Authority figures such as parents, professors, cops (obviously), and anyone else who really should not know that you have taken psychedelics. This is a no-brainer.
3. Alcohol in general. You don't want to be drunk during your first trip (in my opinion, at least, it's a really bad idea), you definitely don't want to be drunk when you make the decision to trip for the first time, and you probably don't really want to hang out with drunk people very much, they're not likely to be very interesting or fun to hang out with. Alcohol and psychedelics don't really mix too well, either in a single person's brain or in a more broad social situation. (By the same token, painkillers, sedatives, anti-anxiety drugs, and pharmaceuticals of various stripes are probably to be avoided as well.)
4. Any situation in which you feel useless, helpless, or otherwise disempowered. This means don't hang around people embroiled in personal dramas (and, of course, don't be embroiled in drama yourself), don't go to anyone's house if you don't trust and like them, don't stick around groups that are doing things you're not into, that sort of thing. You really want to have the ability to do something entirely new and different if at any point your experience seems to be getting unpleasant or even just lackluster. Don't ever put yourself in a position where your hands are tied or you're forced into a claustrophobic situation for any amount of time.
It's all pretty intuitive, really. Do things you like to do, don't do things you don't like to do, and keep yourself open. It's vitally important to be honest with yourself and with others, or else you'll run yourself into a lot of really unpleasant situations. You don't want to go along with something you don't really want to do because your friends are doing it, and you don't want anyone around you to feel pressured to do so either. Look out for your friends.
Oh, and on that note, don't take drugs for the first time unless a good friend of yours is hanging out relatively sober. Once you get a feel for psychedelia, the sober sitter becomes less necessary, but for the first time it's pretty important.
Other miscellaneous tips:
-Turn your phone off. Don't want any calls from the wrong people.
-Don't put more than one or two things into each pocket, or else you're going to have a lot of trouble finding anything.
-Be sure you've got not a lot to do the next day, especially if you're going to consume your drugs after mid-afternoon.
-Tell stories! Tell the story of what's happening to you as it happens, even! Once me and my friend took a lot of drugs and explored some train tracks and we ended up finding a weather machine and an abandoned time travel station. Or at least, that was the story that we were telling and developing as we went along, and tripping in a story that writes itself can be way more fun than tripping in the real world.
-Watch sunrises and sunsets, if the timing is right and you've got a view. Watching a sunrise or a sunset on psychedelics is mind-blowing.
I'm sure I'll think of more, I might going back and editing this post over time. Also, I didn't really address MDMA, which isn't quite the same as traditional psychedelics ... I'm sure I'll end up talking about it eventually, though.