To start, let's look at what causes lip lines. Some of these factors you can control, and some you can't.
One cause of lip lines is, unfortunately, using your lips! Every time you purse your lips to drink through a straw or whistle a tune, you create folds in the skin. Over time, the repetition of this pursing motion can result in permanent creases and lines around your mouth. This is the main reason that people who smoke cigarettes tend to see lip lines develop sooner and more noticeably than nonsmokers do. When you add in the fact that smoking has a drying effect on skin and is known to accelerate the formation of wrinkles and loss of skin tone, it's clear that smoking is a major factor in the formation of lip lines.
Another cause of lip lines is loss of lip volume with aging. As you get older, your skin loses collagen and elastin -- the components of skin that give it a full and smooth appearance. In addition, facial fat, which accounts for some of your lips' plumpness, is lost naturally as you age. Lip lines, as well as other facial lines and wrinkles, become more and more visible as these components are lost.
Another main cause of lip lines is thinning skin. As you age, your skin tends to become thinner and more likely to become creased with use. All facial lines, including lip lines, are more visible as skin becomes thinner.
Aside from giving up smoking, it sounds like there's little you can do to avoid lip lines. There are several things you can do to erase them, however. For more information, read on.