Again, as we said in the MP Conservation section, you should always be seated unless you're actively healing in that moment. Likewise, you should always have a spell potion activated to increase your cast speed. If you want, tell people that you'll donate less force chips to a party because you keep those active.
Buy them from the store or the deluxe spell potions from other rappelz-item.com/tag/player/">players. The best clerics at high levels generally all use deluxe spell potions all the time. We take 30 with us whenever we go into a dungeon. They last 10 minutes each, and you can generally buy them for 3,000 each. So it costs 18k per hour - not bad.
At lower levels buy the normal spell potions from the vendors- they're much cheaper. You should always use one of these, though. The top echelon of healers all do. On Lydian server, I've only seen a small handful that do, though - but they're the absolute best healers on the server. It makes a big difference. Also, really work on getting a feel for when your heal is done recharging. That way you won't try to sit up prematurely.
If you were planning on sitting up and using a lower level heal, but heal isn't done recharging, then just wait for it and use a level10/8/etc heal instead. It's more efficient. It can be tempting to not sit when you only have 2-3 seconds between heals you need to do, but you should sit. It makes a tremendous difference.