I usually follow the sugestions above when fighting druids. Yesterday however, I found myself in a really bad situation against a resto druid in one of those looong duels that occur in some BG's, since the guy NEVER ran out out of mana no matter what i casted.
I did exactly what is said here, constantly spammed Slow, trying and force him to shapeshift in order to drain his mana pool, besides nuking him with a respectable sequence of ABarr - Fireblast - Missile Barrage (whenever procced) while he was silenced, but he easily overhealed all of my burst attempts. To summarize, in the end I was ooM and he didn't kill me, just went away /laughing and /waving to me.
BTW, 1000 spellpower 24% crit 350 resilience mage here, I know I suck and might just got overgeared by him, anyway all I wanted to say is that properly played/geared, resto druids can still be nasty for every class, particularly us mages.