Burn out can be very hard to avoid. My guild just reached 11/12 hard modes in ICC, but rather than dive directly into HLK 25 I made the decision instead to give them a week off, and go for achievements for drakes instead. They are enjoying it immensely (most of the achievements are far easier than hard modes) and they will get a great mount out of it. Even though we raid a relatively lite schedule (9 hours per week), I fully expect that sometime in the next two months (of wiping to HLK 25) it will be best to give them another week of. Pushing hard for progression is all well and good, but pointless if you drive away your guild members in the process (especially if they happen to be your friends).