First things first...if you're not class in able to slow (frostbolt, hamstring, frost shock, entangling roots, really anything a druid can form out of) then this guide is not for you =(
okay, this is really simple, and i'm not sure why people don't d o this all the time but when you slow a druid the normal reaction to that is for them to form out then form back in again. In my resto druid it costs me ~1500 mana to do all that. Most druids (especially feral) don't monitor their mana so they end up going oom quick but since people dont bother slowing them because "they'll just get rid of it" (the debuff) druids don't need to worry. so if you spam slows/polys you can make that druid go oom crazy fast
Calculations:
Feral druid mana: MAX 5K
5000/1500 = he can form out 3 times (not counting the heals and rejuvs he'll probably drop on himself
Resto Druid mana= MAX 10K (if they have like 0 MP5)
10,000/1500= ~7 form outs, not counting heals
Best Classes to do this:
1. Warlocks: Curse of Exhaustion
2. Warriors: Hamstring
3. Mage: Frostbolt (Rank:1), (on emergencies) Polymorph
this tactic might be hard to pull off on a resto druid (even though i had it done to me and it never failed even after using innervate)
hope this helped you guys, because this only happened to my two times.
PS. excuse my crappy spelling, its not me it's because I'm using my laptop >.<
EDIT: i forgot to mention that my calculations are very rough and based on my estimations, the resto calculations are more accurate than my feral calculations because i respeccedas soon as i got to 70, so stop flaming >.<