Make sure you're in a good set and setting. Make sure there's nothing that could send them into a panic (i.e. tripping near a police station or in a sketchy area).
If one of them starts to panic or go into a bad trip, just keep reminding them that everything's okay and they're just on LSD. Try not to sound forceful or condescending, but also try to make sure they're getting the picture. Also, make sure there's no chance of a trip disaster (i.e. parents coming home.)
Make sure there are no "dark" stimulus. Play only relaxing and soothing music, and if one of them starts freaking out about a DOOM poster or something on the wall, remove it.
Make sure one of them doesn't start getting paranoid and try to call a family member or someone who they shouldn't be talking to while tripping.
If one of them starts talking nonsense and looking like they're tripping really hard, don't say anything like "Dude what the fuck are you talking about" or "You're not making any sense," just go with it.
Make sure water and music are readily available.
Also, just make sure they don't do anything stupid. If it's their first times, don't let them "go on a walk" alone or something. If they're planning to obtain more acid than they're going to take, make sure they don't take the rest of their shit while they're tripping. I have friends who have done that.
Again, if it's their first times, try to make sure their doses are on track. 1 or 2 hits each, and no more. However, if they are experienced with acid, it shouldn't be a problem for them to take 3+.
Know that sometimes people on LSD get "weirded out" by sober people (or, more specifically, people not on LSD). So don't take it personally if one of your friends starts acting weird around you.
That's about all I can think of for right now. If you have any other questions, feel free to PM me or hit me up on AIM at T3amAm3rica1.
Good luck and happy tripping!