I'm always painting what I print, mostly for marketing purposes since I sell what I print. But the painted stuff I keep for myself. Usually I always lightly sand the surface, especially PLA, which seems to help for paint to grab on. And I use some sort of spray (Rustoleum or Krylon). Then I take to brush, and in this I find all sorts of paints work well -- acrylic, enamel, and lacquer. Even the metal paints that use an activator to initiate weathering all seem to adhere very well to PLA.
Caution: I had a propane tank model I printed in PLA and it took to glossy white Krylon spray paint very well. Dried well, looked nice. But I wanted an extra glossy look. So I bought some brand of glossy clear coat. That coat shrank as it dried, pulling off the Krylon paint. It looked like a raisin.
Here is a sample of what I do. The blue canister was printed with the FLUX Delta, the others in the background were with Makerbots. The layering lines can be a challenge to hide, but sometimes they can work with the model when painted.