For the softest, easiest look, use lip gloss with or without lip pencil. This gives you the sheerest veil of color, letting your natural lip color come through. Lip gloss over a liquid cheek/lip stain works, too. All you'll have to reapply is the gloss, because the stain stays on until it's washed off.
For a little more color that's still very soft, use a lipstick or lip sheer. Always start in the center of the mouth, working your way out toward the lipline, with soft brush strokes that keep the lipline undefined. A subtle shade's best for this technique -- something that blends into your own lip color at the edges.
For a more defined mouth, first line and fill in the entire lip with pencil, then add lipstick in a matching color. Tip: If you aren't good at staying in the lines with your lipstick, you can avoid the edges as long as your lipstick color matches your pencil color, because you already have a pencil line there.
To create a perfect lip line, take a little foundation and put it just over the edge of your lips, not over your whole mouth. This gives you a clean surface to work with, and creamy foundation helps pencils glide easily.
Then, with a lip pencil, go from each corner of the mouth toward the center. Follow these four points (pick any order and stick with it): top left to center, top right to center, bottom left to center, bottom right to center. Steady your hand if you need to. This "four point" method is the easiest way to be symmetrical (whether your mouth is or not).
Once the outline's penciled in, fill in your whole mouth with pencil. Filling in gives you an even layer of color across the lips and much longer wear. After filling in, take your lip brush and go all over the lip to make sure the pencil color's evenly placed.
For really amazing wear, set the layer of lip pencil with a dusting of powder (dust right over the pencil, or put a tissue over the lips and dust over that). Then, you can apply a cream lipstick or a gloss on top of the pencil.