The saying goes, "Check you birthday suit on your birthday." Take some private time with your skin, a mirror, and no clothing to look for changes in moles and freckles as well as to notice any new ones. Follow the ABCDE's of skin care-- you need to keep an eye on (or get your dermatologist's eye on) any pigmented area on your hide that falls into one of these categories:
Asymmetrical- Any mole or freckle that has an uneven, lopsided shape or is different on one side.
Border- Irregular edges.
Color- Moles that are blue, brown, black on one side but not the other or that have changed color in general.
Diameter- Larger than a pencil eraser.
Evolving- Any mole, freckle, or lesion that is changing or elevated.
The Skin Cancer Foundation has a really nice page on self-examination.
There are several good websites about skin cancer, assessment, treatment, and prevention:
Yahoo Health has nice info on soothing a sunburn should that happen. I've always favored the pure aloe gel and soaky bath with vinegar.
Anything that comes between you and the sun offers some degree of protection. I keep a big dark golf umbrella in my car, just in case I'm stuck in the sun some place.
Coolibar offers protective sun clothing, as do SunProtective Clothing and Sun-Togs.
Sunscreen? Anything is better than nothing, but some are better than others. American Academy of Dermatology has a nice page about sunscreen. My personal derm-lady insists on sunscreens that contain zinc oxide or titanium dioxide.