Someone today asked me how to paint a room. 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.
I'm sure I've forgotten something, but it seems that there's always someone out there painting a room for the first time and they all seem to have the same problems.