Over the years I've noticed that there are 3 quick, free and easy things that most experienced painters refuse to do that holds their quality back from producing professional results:
1. Caulk with a wet finger or rag, prior to painting. This reduces drag, leaving a smoother finish on the bead of caulk. It also prevents building texture with drag lines and tiny chunks of semi-dry caulk. The caulking deficiencies only look worse once painted, so proper application will give a better paint finish. Caulking applied after the painting is done will eventually collect dust, discolour and look unprofessional in a short time. Caulking requires top-coating with paint...
2. Filter the paint. Low VOC latex coatings are notorious for forming chunks in the paint. Even $50 gallons of premium paint can be full of tiny solidified bits that end up on the wall. Pouring paint through a strainer saves time compared to picking chunks out as you apply paint. This is especially important for the final coat, or on mid to higher end projects.
3. Sand between coats. I know most painters are allergic to painting, but this step really helps reduce texture build up and makes for nice smooth walls. This step is critical in new construction. I would add the cleaning of new drywall prior to priming as another important step.
Do these three things and your results will stand out from the average painter.