One thing that I've found useful is having a Waymark in the hotbar.
This way while doing big pulls I place the Waymark to announce "this is where I will stop".
Its specially useful for SCH, since their Shadow Flare is a pain to missplace, also WHM and AST gets a chance to time up their holy spam or card deployment.
About swapping to DPS stance, its not really necessary on trash pulls (or at all), but if you want to, have Rampart at hand and use it as a sub-tank-stance. So you swap to DPS, pop Rampart, and return to Tank stance once Rampart runs out.
Personally the easiest Tank to test waters for swapping is the WAR. PLD is a tad more annoying so I usually only do that at bosses.
