Don’t wear sunglasses in the sun since this blocks melatonin production. Melatonin in the skin turns skin darker, so if production is blocked by wearing sunglasses, you are more likely to burn than to tan.
Of course it is fine to wear sunglasses when driving, for instance, since the useful part of sunlight is cut out by glass.
Also if a sunblock of a higher factor than 8 is used, then vitamin D synthesis is reduced by 95%.
Suntans need building up slowly to avoid burning. Most sunscreens are carninogenic. I recommend looking at the site www.organicwish.com for a recipe for home made non-carcinocenic suntan sunscreen.