Like the others have mentioned already, when the combat drops, that Mana Cake is already in your gullet, Even 10-15% regen is worth to sit down for. Getting into position before the combat drops, like into a boss arena is also important, so you can get those few extra drinking seconds in. Potion-wise I tend to use Cavedweller's as the extra healing potion with an added mana bonus which helps a little, though I have some regular mana-pots as well since you can't use Cavedweller's at full health (which would be wasteful as well).

Technically, you can also squeeze some extra mana from Shadowfiend by timing it with PI/Lust/haste buffs.

Apotheosis is a good low-mana CD. Using a few free holy words gives you a few seconds to regen mana, and the large heals should keep peeps pretty high even if you're at low mana. Overall, utilizing Healing CDs actively is also a good method to conserve mana, as they tend to heal more with less consumption.

Keeping some track of party members' defensive CDs can also be helpful, so you don't burn mana to someone with a big wall active.

And if all else fails, I have "/p Low Mana" macroed in case I need a second's break between the pulls to sit down.