You have prepped the surfaces and chosen the right paint and applicators and now the fun starts. Before applying paint, be sure that it is properly mixed. This process assures that there are no mismatches among different cans of paints. Mix all your paint into one large container until the paint's color and consistency are uniform. It is important to prepare enough paint to cover all surfaces with this mix, since matching can be difficult if you run out.
Air often causes a scum on oil-based paints.
Also, if you are thinning paint with either a thinner for oil based paints or water for water based, thin slowly so as not to over-thin and thereby require adding more paint. Finally, use a nail and hammer to punch a hole in the rim of the can so that excess paint will drip back in. By painting the ceilings first you can be sure that any drips falling on the walls will be covered. When painting the walls, always paint from the top down, again to be sure drips are covered. And, finally, do the trim so that any paint that accidentally gets on the trim can be covered.