[Gello]

Another trick is if you shift+click a set (in the menu or on the bar), if you're currently wearing that set it will unequip it. If you're not wearing that set it will equip it.

Say for instance you have these sets:
Nature Resist - bracer, cloak, boots, legs
Fire Resist - wrist, bp, helm, rings
PVP - most of a set
Tanking - most of a set
DPS - most of a set

When you zone into ZG: equip 'Tanking'
Before bat priestess: equip 'Fire Resist'
When done with bat priestess: shift+click 'Fire Resist' to unequip it (back to whatever was in those slots (Tanking) prior to equipping it)
Before snake priest: equip 'Nature Resist'
When done with snake priest: shift+click 'Nature Resist' to unequip it.

There's a new option in there also called 'Auto Toggle Sets' that does this without the shift.

If you select Tanking and you're not wearing all of Tanking, it equips Tanking.
If you select Fire Resist and you're not wearing all of Fire Resist, it equips Fire Resist.
If you select Fire Resist again and you're wearing all of Fire Resist, it unequips it.
etc

edit: Also, the unequip doesn't require a set be the last one equipped either. If you use for weapon sets (I do), you can do this:

Equip 'Tanking'
Equip '2h'
Equip '1h+shield'
Equip '2h'
Equip 'Nature Resist'
Equip '1h+shield'
Equip '2h'
Unequip 'Nature Resist' <- should unequip just the 'Nature Resist' slots irregardless of what happened since.