After painting the inside of our ~2600sqft multi-family property, and also 2 1200sqft apartments in the past year:
Buy quality paint - for our property, I ended up spending more for Benjamin Moore Regal Select vs. Behr at Home Depot like we did for the apartments. It was ~$15/gallon more expensive, but the paint seemed much thicker and we were able to get everything coated with 1 coat, optional 2nd in a few areas. With the Behr, it was basically 2 coats required, optional 3rd. This saved us a ton of time, and there were also less drips/splatter.
Buy quality roller covers - I've been using the Purdy and Wooster covers and they seem to hold the paint well. They are ~$12 for 4 in my area.
Buy quality brushes and wash/reuse them - I also recommend Purdy or Wooster for this. Lowes has frequently had a 3 pack of a 1", 1.5", and 2" for ~$18, which was basically like buying 2 getting 1 free.
If you're doing a lot of trim, pick up one of the small Wooster/Purdy rollers. I roll the faces of the trim, then brush it in to get the right texture, but it saves a lot of time. This is especially helpful if you're just freshening up the paint vs. changing the color. After you roll the faces, you may not even need to cut in along the edges.
Lastly - make sure you clean the area decently well. In one of the apartments we painted, we realized we didn't even need to paint the trim after a good scrub!