Deciding what to cut and what to keep is one of the hardest parts of deck building. There are usually so many good cards that you want to include it can be very hard. If you're making a balanced deck (one that includes a similar amount of offense and defense) then try to stick with 8 Heroes, 8 Good Enhancements, 8 Evil Characters and 8 Evil Enhancements.
As you decide which ones to keep and which ones to cut start by deciding which Heroes are most important to your offense. Do this until you have chosen 8, then put the rest of them aside. Do the same for your Good Enhancements, Evil Characters and Evil Enhancements. When you're all done this will give you 36 cards. Add in 7 Lost Souls and your at 43. This leaves you room to add as many dominants as you have, and a few sites/fortresses/covenants/curses and artifacts.