Playing the Slots--Enhancement Slots, That Is
One of the most important and yet most difficult parts of building your hero is deciding how many Enhancement Slots each power needs. I certainly haven't perfected it, by any means; early on in levels, I just go for the easy solution of making each power set have a certain number of slots for each power within that set. (Take my low-level Ice/Energy Tank: Each Ice Armor power currently has 3 slots, and each Energy Melee power has 2 slots.) But as my characters level up, I discover which powers need a bit more enhancement and put in additional slots as necessary.
But the task of slotting Enhancements into powers is a lot more complicated than this, especially if you want to maximize your character's benefits, and ESPECIALLY as you get into higher levels and/or start slotting your character with Enhancement Sets instead of just basic Invented Origin stuff. For instance, the number of slots on Healing Aura can matter greatly if you are lucky enough to have the full six-enhancement set of Miracle to slot in it. If you don't have enough slots in Healing Aura to take the full Miracle set, you won't get all its bonuses.
To ward off such tragedies as the one outlined above, think carefully about each enhancement slot you place. Don't worry about symmetry within the set; for instance, if a power needs more than one Endurance Reduction to make it function better, then you need an additional slot. Attack powers might need more than one slot to accommodate a Damage, Accuracy, and Recharge Reduction; defensive toggle powers might need an Endurance Reduction or two if the toggle drains you too much during battle. Remember the list of enhancements and what they do, and think about which types of Enhancements might help out your powers. Sometimes it all depends on whether you want a power to do more damage, cost less to cast, hold more effectively, recharge faster, etc.; especially in early levels when enhancement slots are at a premium, prioritize which effect of a power needs to be boosted first. With each odd-numbered level (beginning with 3), you get 2 enhancement slots to place on whichever powers you wish to pump up with extra Enhancements. Also, after level 30, you start receiving 3 enhancement slots with each odd-numbered level!
What If I've Messed Up My Slots?
Your recourse if you discover you have misappropriated slots is to do a respec on your character. Learn more about respecs here.