About once a month, someone asks me how to paint a room because they think I'm "handy." I'm not a professional painter. I don't even like painting much. But I have gotten pretty fast at it over the years. I've ignored lots of rules and found lots of others that work. And I've made a huge number of mistakes. Even after 12 years of painting things, I still can't make my paint jobs look as good as the ones that pros do, but I can do it for less. That said, when the work requires more than 5 days of effort on my part, I look to hire it out -- pros can generally do that sort of thing in a lot less time and do it better.
A couple of rules of thumb before I start on my tips and tricks. I generally figure that a gallon of paint will cover about 200 square feet twice (two coats). I count on doing two coats of everything because I can always see through the first coat (even when the pros do it). I generally figure that I can do a 15 x 15 room in a day by myself. I use a dehumidifier to speed the paint drying. I don't paint when it's below 50 degrees, above 80 degrees, or above 60% humidity unless I have a dehumidifier, a heater, and/or an air conditioner. I listen to what the people at the paint store tell me. I don't listen to what random people at Home Depot tell me; I do listen to the ones who work in the paint department because several of them are also professional painters.