I probably sound like a terrible novice, but here's another painting tip I've stumbled upon -- get a caulking gun. A friend of mine was kind enough to lend me one and it has proved to be the MVP of my recent painting endeavours.
I've gone through approximately 474 tubes of caulk. I've been caulking everything -- and if you hit every spot and crack with the stuff before you paint, you'll have nothing but a solid, smooth finish in the end. You'll sit back and stare at your walls and trim with a deep pride that goes on for at least 48 hours. After that, it does wear off.
But honestly, if you skip this step, every crack or separation in your moulding or trim will show up as noticable as a Hershey bar in the snow.
I'm new to this caulk gun stuff -- but it's addictive and I'll never go back to slapping paint over untreated surfaces again.