Know your walls. Are they drywall or plaster? Plaster absorbs more paint and the color often changes.
Fix the cracks. Make sure the surfaces are clean and dry before you paint.
Try several samples. Prime the walls and then apply a square foot of paint in several places in the room. Or paint pieces of poster board and tack them up around the room.
Try the color near the trim. That is where the color will "pop." Check back to see how the paint looks in different kinds of sunlight in the day and in artificial light at night.
Take time to prime. That ensures an even color. And don't start a new can of paint in the middle of a wall. Even the smallest difference may be noticeable.