The way I did this was to find out what I needed to concentrate on the most. Since my primary concerns were Power and AC, I cast all the buffs that raise this as high as possible. This is what I do for soloing.
For groups, it's rather situational. If you have a lot of fighters or melee types, then you will want buffs that raise Strength, Stamina, and Agility, and Health. For a mostly caster/healer group, you will still want to buff at least some of those stats (to make the tank more effective) but you will also want buffs that raise Intelligence, Wisdom, and Power. If you are the only healer, and are with an otherwise more or less even combination of classes, then your best bet is to concentrate on your own Power and Wisdom and fill out whatever needs the most. If you are backup, you will want to either buff the main healer's stats (Wisdom and Power of course) or complement his buffs (if you are the same subclass, you can cast whatever buffs he left out due to the concentration limit which is a good way to get around the limit).
When in doubt, don't be afraid to inspect other players. This can give you a lot of information about what weaknesses they have that you should cover and what buffs can correspond to what.
Never be afraid to at least recommend who should tank. You are the healer and you will make or break most groups. If your group fails to respect you on that part, then more than likely it will lead to a wipeout. As long as you are confident of your abilities, no one will ever turn you down for an invite. Out of all the classes in the game, healers are probably the most wanted even beyond enchanters. If you feel like the group is not listening, then make the decision righ there wether or not it's worth the possible experience debt.