This is a guide to Tripsitting online, primarily guided around doing so in TripSit's #tripsit channel, though most tips should be transferable to most tripsitting scenarios.
Greet users with a simple, calm and neutral phrase such as "hello, how are you?" If the user requires some help, make sure you first gain from them an understanding of exactly what drug(s) they are on, at what dosages and how long it was since they took them (always better to ask the user to list all of the drugs he is on even though the problem seems to only be coming from one of them).
If a user is online, and not getting too much into convo, then don't idle- they might think that no one is listening to them and leave. When possible, try to greet the user as soon as they enter the room (you can use the ~gettripsitentries command to help with this!).
The most important thing to keep in mind is that panic attacks are not dangerous. They usually originate from a negative thought loop and can be stopped by reminding the 'victim' that they are in no danger whatsoever. Usually a short period of breathing exercise (30s) will be able to 'break the cycle' in terms of the thought loop and you will be able to move them onto another subject. Ask the user to shut their eyes, breathe slowly and deeply and clear their mind of everything but their breathing for thirty seconds - and reassure them that once they have done this they will be feeling much better.
Remember, panic attacks are uncomfortable but not dangerous. Failing being able to perform meditation, suggest a change of setting - this can be done most easily by suggesting the user goes to get a glass of water.
Ascertain whether just panic attack, and beyond that recommend the hospital. We aren't doctors. You should be able to do some fast googling to identify whether a certain substance at a certain dose or a certain combination is safe or not. Don't freak out trippers by saying they may be in danger when they're on, for example, ~1500ug of 25-I-NBOMeN-Benzyl-Ortho-Methoxy derivatives of psychedelic phenethylamines. (high dose but not dangerous). Get some background from the tripper; 4g of paracetamol won't harm most of the people but if someone already has problems with their liver it is really dangerous.
Use your imagination to find something relevant that the tripper finds interesting. Engage them in a subject they find interesting! The most useful defence a tripper can have against bad trips is the ability to change the setting. Changing the music, the lighting, the room or just about anything can have a huge influence on the mindset of a person who is tripping and this makes it an invaluable technique. Consider also that challenging moments in a trip can be an incredible opportunity for the development of one's character and you may come to the conclusion that many others have - that benzos are great but really ought to be a last resort.
You can draw a parallel outside of the context of illegal drug use to those that drink heavily when times get tough. Instead of dealing with the hurdles that life presents they retreat to the bar, and they tend to be very immature as a result.
There is a wealth of useful information which can help provide you and a tripsitee with information on our Wiki. You can either browse the Wiki itself at http://wiki.tripsit.me, or various important articles are also stored in tripbot quotes such as ~stimulant comedowns. You can get bite-sized information about certain drugs and classes of drug in-channel with the '~drug drugname' command. For example, '~drug mdma dose'. You can also get a link to a nice web-page with all the information about a single drug with, for example: ~factsheet 2cb.
Erowid is about the best source of objective drug information on the web. You can find good answers to a lot of questions on this site! You can link to Erowid directly from tripbot, for example with '!erowid dmt dosage'.