With Patch 3.0.8, Rank 8 will reduce the damage taken from area of effect attacks by 50% for 6 seconds in addition to its existing effects (Feint will still need a target in melee range).
Sleight of Hand increases the threat reduction of feint by 10% per point, for a total of 20%.
Glyph of Feint reduces its energy cost by 10.
The 5 piece bonus of Bloodfang Armor improves the threat reduction of feint by 25%.
Tips and tactics
Ideally, feint should be used when you are approaching the tank's threat on a mob. Feint allows rogues to continue dpsing when classes without a similar ability would have to stop and wait for a tank to get more threat. If you have an addon that keeps track of how much threat you and the tank have, you can tell when your threat level is getting dangerously near to the tank's threat. If you don't have such an addon, you should at least keep track if you overaggro a mob, and use feint if you do.
An alternative to using feint in raids during longer boss fights is to wait until the boss is down to ~60/70% health, and then vanish to completely dump your current threat. The threat dump from Vanish gives the tank enough of a lead on threat so that you can continue dealing damage without using energy to feint.
At lower levels, Feint may be dismissed as a useless ability due to the negligible amount of threat removed. At higher ranks, the threat reduction becomes more significant. Nevertheless, the threat reduction of Feint does not scale with the damage output of the rogues. As a result, most raid level rogues find it a very inefficient threat reduction ability and prefer not to use it unless necessary.
Try to keep track of how much threat each party member is drawing from a mob when using Feint. The threat you are removing from yourself may be the only thing keeping a mob from attacking the party healer.