Mix water-base paints only with water-base paints, and oil-base paints only with oil-base paints. You cannot mix water-base paints with oil-base paints, as they will separate.
Stay with the same brand and line of paint. Different companies use different solvents and additives, and mixing them together might cause a streaky appearance.
Stir thoroughly. You will achieve best results by using a paint mixer attachment on your power drill.
Blend your paints in a large, clean bucket. Add only a little at a time, slowly combining paint sheens to be sure you get the right mix.
Test your mixture frequently as you combine the paint. Brush a few strokes onto the wall to see if you have achieved the desired sheen.
Mix enough paint to complete your entire job. It will be difficult to exactly duplicate your mixture if you run out of paint.