What Bodhi said is, of course, all good advice. Some things I'd add:
Judge once a minute, at least. You should have Judgements of the Pure, which gives your haste a kick in the pants. It matters little /what/ you judge currently (exception: don't judge light if you have a Ret pally in the raid, since she'll be doing that and it's much better than yours). And it really won't matter at all with 3.2. Also with 3.2, you'll get a mana return if Wisdom has been judged on your target.
As for getting sniped, how is the raid set up? Do you have an explicit assignment, or is it free-for-all? Are you assigned to (a) tank, or are you another raid healer? If you're watching the raid, for whatever reason, you'll get beaten a lot. You have one instant heal, on a six second cooldown. You have no smart heals. You have no HoTs. You have one multi-target spell - if you're glyphed for it - and it's very limited. If there is a priest or tree around, then by the time you notice a health bar dropping, the person is already being healed via HoT, or there's a smart heal already jumping to them.
Basically, you shouldn't be racing to heal raid damage, for whatever reason. Ideally, as Bodhi said, you should be focused on the single target taking the most damage - i.e. the main tank (with 3.2, plus possibly a second target, presumably an offtank). Your strength is efficient healing of massive amounts of damage, for a long time. Each fight has a rhythm to it, ebbs and flows of damage (until you get to hardmodes, where I hear that it basically just flows). Work on learning the rhythms and using that to dictate your strategy moment to moment.
If the tank lives, you did your job. Then you can start adding wrinkles: throwing HS around to help with raid damage, using Beacon on the tank at safe times so that a FoL or HL on the raid does double duty, using your Hand spells when someone slips and draws too much attention, using Divine Sacrifice/Guardian to best shield the raid as a whole.