It depends on the kind of proc.
General rule :
Damage and procs that affect enemies, in low damage AoEs first and then low damage single target if no AoE low damage to slot it in. The same goes for when I'm slotting defense debuff, slow, stun/disorient, and to-hit debuff enhancements of any kind.
Procs that buff you, in powers that are single target that recharge quickly to give them more chances to go off.
As far as sets, I rarely use whole sets.
There is often a sweet point to break a set over two or more powers - especially the archetype enhancements.
I use
Health - Numina + Recovery/regen, Panacea +HP/+endurance (and sometimes a Preventative medicine - absorb proc)
Stamina - Power Transfer +HP and and performance shifter +Endurance
Accurate Heal - Touch of the Nictus - chance for negative damage
Defense - Luck of the gambler - increased global recharge speed, Reactive defense - scaling damage resistance, karma - knockback protection
defense debuff : Achilles's Heel - chance for -Res - defense debuff set, Analyse Weakness +to hit
fear : Glimpse of the Abyss - chance for Psi damage.
Hold : Lockdown +2 Mag hold
knockback : forced feedback + recharge
Pets that stay close - Sovereign Right - RES bonus pets 40', Expedient Reinforcements - RES bonus pets 20'
Range AOE - Annihilation - chance for -Res, Bombardment - chance for fire damage, Positron's blast - chance for energy damage
Range single target - Devastation - chance for hold
Resist - Aegis - Psionic/Status protection, (if tank) Unbreakable+7.5 max hit points, Gladiator armor - Teleport protection/+Def (all)
Slow - Pacing of the Turtle - recharge slow
Taunt - perfect zinger - chance for Psionic damage
to-hit : Gaussian Fire Control - build up
to-hit debuff : Dark Watcher's Despair - recharge slow - to-hit debuf set.
Procs that I don't use and suggest not to use
Experienced Marksman - Snipe - quick snipe (you can never do a true sniper shot when this is slotted)
Ghost Widow's Embrace - Hold - chance of Psi-damage (psi damage is good, but [I believe] you are better up slotting up the hold duration and/or recharge on hold powers)
Procs that add smashing or lethal damage (most enemy types have a good about of defense/resistance versus both smashing and lethal)
You can always use unslotters to pop enhancements that you want to swap out and back into your enhancement tray.