Interior House Painting Tips & Painting Techniques for the Perfect Paint Job

From TheFamilyHandyman.com

To avoid lap marks, roll the full height of the wall and keep a wet edge

Roll the full height of the wall 

Lap marks are those ugly stripes caused by uneven layers of paint buildup. They occur when you roll over paint that’s already partly dry. (In warm, dry conditions, latex paint can begin to stiffen in less than a minute!) The key to avoiding lap marks is to maintain a “wet edge,” so each stroke of your roller overlaps the previous stroke before the paint can begin to dry.

To maintain a wet edge  start near a corner and run the roller up and down the full height of the wall, moving over slightly with each stroke. Move backward where necessary to even out thick spots or runs. Don’t let the roller become nearly dry; reload it often so that it’s always at least half loaded. Keep the open side of the roller frame facing the area that’s already painted. That puts less pressure on the open side of the roller, so you’re less likely to leave paint ridges