Have you notice that you are squinting a lot lately? Are people telling you to "stop squinting" often? Do all the newer photos of you show you squinting?
Get an eye exam. If it's been more than one or two years since the last exam - it's time. People start to squint for a couple of reasons: to "help" focus and to reduce bright light (sunshine, etc.). Chances are that you could benefit from wearing glasses or contact lenses.
Wear your lenses / glasses. Keep glasses close by so that straining (and squinting) is minimized. Consider a bifocal lens if you find you are constantly wearing and removing glasses for different situations.
Change locations. If you squint because of difficulty focusing, move closer to or further away from the subject whenever possible (ie: movie theater, classroom, etc.).
Wear sunglasses. If you are squinting mainly outdoors in the sunshine, a pair of quality sunglasses may solve or reduce the problem. Make sure the glasses block at least 99% of ultraviolet (UV) rays.
Relax. Be aware of the times that you squint. Recognize the situations that cause you to squint: bright sun light, nighttime, in movie theaters, looking at close or distant objects, etc., and relax your eyes. Ask friends to tell you when your are squinting to help identify the triggers of squinting.
Sunglasses "do the squinting" for you by reducing light. This allows the eyelids and pupils to remain open. Since the eyelids and pupils protect eye and the optic nerve, cheap sunglasses that do not block UV rays "trick" the eyelids and pupils to allow those damaging rays to enter the eye itself. Over time, these rays can cause damage to you eyes.