Knock these to-dos off your list to ensure that your insulation will last for years and keep conditioned air where it belongs: inside your house.
1. Fix roof leaks. Water is insulation's enemy. It creates a breeding ground for mold and mildew and ruins the air-trapping pockets that block heat flow. Look for water stains on the roof sheathing or damp or moldy spots on attic joists and existing insulation as a clue to where leaks might be.
2. Box out light fixtures. Unless you're using mineral-wool insulation or your fixtures are rated safe for contact with any type of insulation, don't allow the material to touch or cover recessed cans or lights from the floor below—it's a fire hazard. Use hardware cloth, metal flashing, or scrap plywood to create a safety gap of at least 3 inches all around fixtures.
3. Direct all exhaust fans and vents to the exterior. Though it's against building code to vent any kind of exhaust to an attic space, many home builders have gotten away with this shortcut. Correct that mistake so that humid exhaust air doesn't get trapped in your insulation and ruin it.