Taking time to perform a thorough surface prep and painting in an orderly systematic way are the keys to getting professional looking results when doing any paint project. Skimping on the prep work is probably the biggest mistake people make when painting. Take the time and do it right, it really does affect the end result. First carefully inspect walls for cracks holes dense or other surface imperfections before priming or painting. Use a lightweight spackling compound in putty knife to fill and repair any holes or imperfections, then remove any excess spackling with the putty knife and allow the area to dry completely. Once dry, use a small piece of very fine 220 grit sandpaper or a sanding sponge to smooth the repaired areas flush with the surface. Wipe the walls clean with a damp towel or sponge and allow them to dry before priming or painting. You should also make sure the walls are clean and free from dust. You’d be surprised how much dust actually builds up on walls overtime. Out-of-the-way corners and areas behind furniture can also have lots of cobwebs. Use something like a floor duster to wipe the walls clean to ensure paint applies evenly.