The lasso tool itself won't do that. Switch over to the magnetic lasso (it's hidden under the lasso tool icon, click the tiny arrow to reveal the other tools). If there's pretty good contrast between the lip edges and the area next to the lips, the magnetic lasso should find the edges for you. It will "stick" to that edge.
If you are still have trouble, try using the Pen tool. It takes time to learn how to use the Pen tool, but it allows you to make much more precise selections. E-mail me through my profile if you need more help.
