First of all you need a newly-sharpened lip-liner. It should be about the same color as your lipstick, maybe a just little darker. Draw a line right on the outer contour of the lip – not inside it. Some women can actually try and draw the line outside the lip contour - see if it works for you without looking unnatural.
Put on lipstick. If you want a more discreet color, apply another, lighter tone on top of the underlying one. I sometimes even use two lipliners on top of each other just to get exactly the color I want.
Put some lipgloss on the center of the lower lip to get a highlight and make the lip look bigger. Just make sure the transition between the highlight and the areas around it are smoothed out so that it looks natural.
Add some highlighter right above the upper lip.
If your lips look too glossy, "bite off" against a piece of paper.