Right side of LT/RT for instant cast actions, to make movement easier. Left side for things with cast timers and Scenic Muse, which has a long cooldown.

RT, RT for actions that are used less frequently (or that I haven't unlocked and don't have a home for yet).
LT, LT is the standard layout for my ranged/caster DPS classes. 

Blank spots are for things I use frequently but can't assign here (wedding ring teleportation, preferred mount, etc.

Pictomancer

mungo_fungo

SMN setup that took me to 90. Do I have Swiftcast all over the place? Sure, but that's because when I need it I hate scrambling for it. For the blank spots, I'll usually put in things that I use often out of combat (Eternal Bonding ring, treasure maps, WT, ect.)