One strategy that I haven't seen abused very often is the ability of Yawn to effectively cripple opposing threats and force switches consistently in battle.
My Defensive Swampert which has consistently been a pivotal part in my Gym victories utilizes the following set:
Swampert
@ Leftovers Careful Nature
252 HP/40DEF/216SPD
Yawn
Curse
Earthquake
Waterfall
Basically I switch into a resisted or neutral attack, Yawn to cripple or force to switch. Then if possible Curse on the switch and proceed to create a hard hitting tank.