You wouldn't put make-up on a muddy face, would you? Stripping wallpaper, filling holes, sanding rough spots, and wiping down with a tack cloth will insure a better finish every time. Preparation is the most tedious part of the whole project, but this is what separates the sloppy from the sleek.
A good primer is almost always worth the money and time. I recommend Kilz brand. One coat of primer and one coat of paint often are better than two coats of paint.
If it's a kitchen or bath, buy kitchen and bath paint. Research, ask professionals, and ask the person assisting you at the paint counter. In my humble opinion, for walls, I love eggshell finish in most brands. It is usually able to be wiped clean. Eggshell reflects some light, but still does not highlight every imperfection like semi-gloss or gloss. It also does not absorb oils and stains like a flat finish.
I learned this lesson the hard way. After cutting in (painting the edges of) five spaces with cheap, straight-edged brushes, I felt I was just bad at the whole painting thing. My mom recommended a Purdy brand angled brush. Voila! I felt like a pro! I've since discovered that other brands are wonderful as well. I like the 1 1/2 inch angled brush best. For me, a proper brush eliminated the need for taping off. If you are new to painting, I would still recommend painters tape, but the right brushes make a much cleaner edge.
Have you ever gone into a home and thought to yourself, "Good grief! Did the owner color match their paint to a box of magic markers?"More often than not, the intentions were a subtler hue. When you paint four walls in one shade, the result is more intensity. I have never heard a homeowner say, "I thought this would turn out brighter, but I'll have to settle for this washed-out look, I guess."In this realm,"Less is more." Thanks for the quote again, Robert Browning.