Don't misuse a ladder. For example, don't set it up horizontally as a scaffold unless it's an articulating multiuse ladder that has that feature. Use a stepladder only in its fully open, A-shaped position, making sure the spreaders are locked. Beware of pinch points--any place where parts come together. When closing, folding, or retracting an extension ladder, grip the sides of the ladder securely, keeping your hands clear of the descending sections. And don't jury-rig equipment, say, by splicing two short ladders to create a longer ladder.
Place ladders on a firm, level surface, using the leg-levelers if necessary. Don't put lumber, a rock, or other object under a ladder leg for leveling. When raising an extension ladder, lock each section securely before going on to the next. With any ladder, look out for power lines, and watch for hazards to people in the area. Try to avoid setting up near a doorway or other high-traffic area. Outdoors, check the work area from the ground with a pair of binoculars to make sure an insect or bird's nest isn't there.
Lean a straight or extension ladder against a wall or other flat, fixed object, not against a narrow tree or other surface that can't support both side rails. Position the base of an extension ladder 1 foot away from the wall for every 4 feet of height--that's 3 feet at the base for a 12-foot ladder, or roughly a 75-degree angle. Shallower angles increase the chance that the ladder's feet will slide out from under you.