Using a lip liner will make your lipstick or gloss last longer. When choosing a lip liner get one that matches your natural lip color like MAC's Whirl or Subculture. By doing this you can use one lip liner with different lipstick colors. Also, using a color that matches your lips makes it easier to line a little outside of your lips to make them appear fuller.
When you line your lips, line and then fill them in with the lip liner. This will do two things: make your lip color stay in place longer, and keep you from having an outline of lip liner around your lips...blending is key!