Always do whatever side-quests you can find on a planet before leaving for the next one. Most planets should have a "bonus series" that unlock after you have completed the planet's main side-quest chain (and probably your class story chain too), although in some cases these are not intended to be undertaken until later (an example for Empire players being Nar Shaddaa, whose bonus series quests are aimed at level 31 - i.e. post Alderaan, 2 planets later).
Your main story quest is typically a level or two above the side-quests you'll be doing at the time, so be cautious about rushing straight into that. Best to run through some side-quests first and continue to intersperse side-quests with story quests as you move around the planet to keep yourself at or above the level progression curve.
Common courtesy suggests you should apply your class-specific buff to every friendly player you see. Hopefully they will reciprocate, meaning if you buff players from the other 3 classes (on post-starter planets) you should end up with all 4 buffs applied to you (strength, endurance, willpower, critical % I believe). If you have heals, it's always good to heal other players nearby, especially if they are in trouble. Be aware that healing/buffing someone who is in combat will flag you as hostile to whatever they are fighting. At the very least this will put you in a combat stance for the duration of that player's fight, killing any non-combat abilities like Sprint.
Generally it is best not to directly interfere in someone else's combat (i.e. engage their enemies), unless they are fighting an elite or something and getting their ass handed to them or if they specifically ask for help. If you see someone waiting around for one of the elites that form the final part of most bonus quest lines to spawn and you are after the same mob, invite that person to group with you (or accept their invite if they offer). That way you'll help each other out and both get the quest item to finish the quest chain.
Last edited by DarthParametric; 12-19-2011 at 02:14 PM.