Not all lip balm is created equal. If your lips have passed from arid wasteland to full-fledged desert, you're going to need a balm that provides extra moisture protection. So don't just buy any old one you happen to see in the checkout aisle at the grocery store -- not that those products aren't what you're looking for -- but if your lips are severely damaged, you'll need to peruse the ingredients list before you can tell for sure.
Apart from an SPF of at least 15, beeswax, phenol and petroleum jelly are usually good ingredients to spot on the list. (If you see soft paraffin or petrolatum, those are basically synonymous with petroleum jelly.) If your lips are particularly painful, you may want to grab something that also has ingredients like camphor and menthol to help provide some relief.
Don't be afraid to reapply frequently, although sparingly is best -- otherwise you might be tempted to lick off any excess. Putting on lip balm before you go to bed can also be a good idea, especially if you tend to breathe through your mouth when you sleep.