Other Things to Consider
If you know someone who has recently hired a painting contractor and they were happy with the job, strongly consider using that contractor. Ask your friend if there were any problems at all (e.g., they were never out of the house on time, which interfered with their dinner, and ask the contractor to make sure that won't happen).
If a painter offers to do the job for time and materials, just say no. This almost always will never work out to your advantage. It's always better to know the cost up front.
Make sure the painter will use the highest quality paint. You may pay more for this, but it will last longer and look a lot better. You can verify they did this either by looking at the label on the paint can or asking for a receipt.
Things to Keep in Mind When the Job Starts
Move any prized or breakable items out of the way. Contractors are respectful of your possessions, but making sure valuables aren't harmed is your responsibility as well as the contractors, so do your part.
If you have a preference where the contractor and his or her crew smoke or eat, let them know. Don't expect them to read your mind. If you would rather they used your kitchen than your patio, let them know.
Don't blame the contractor for things beyond his or her control. A contractor can't know in advance what is underneath the wallpaper. There could be numerous layers that require extra time to remove. Walls could need more repairs than anticipated. If you chose the color and then hate it, that's not the contractors fault. Contractors are responsible for applying paint, stains, and varnishes correctly, not for the choices you made. Contractors will almost always get a small sample, apply some, and then let you see what you think before buying enough for the whole room or house.
People respond to each other. If you are respectful of the contractor, pay on time, do your part, and communicate well, the crew will appreciate your efforts. It's certainly not required, but taking the time to make sure cold water is available or even supplying donuts will certainly keep the crew happy.
Don't skimp on materials. This bears repeating. Pay for the best materials. They last, they work, and quality can never be faked.