Look for chipped or peeling paint. Remove it with a paint scraper to create an even and smooth surface for the paint.
Fill a bucket with warm water and a few drops of dish detergent. Use a lint-free cloth to wash the walls clean of dirt, oils and dust. This is one of the biggest factors in preventing semi-gloss streaking.
Prime your wall with a latex primer. This will create an even coat of color and help the paint adhere and avoiding streaking.
Add latex paint conditioner to the paint to avoid brush streaks. Occasionally the paint may be too thick, and brush streaking will occur. A latex paint conditioner will help the paint spread evenly on your walls.