Myth No. 1: Last year's sunscreen is still good
Unlike a fine wine, sunscreen does not improve with age.
In fact, sunscreen goes down hill pretty quickly, so if you have some leftover from last year, chuck it and spend a couple bucks and buy a new bottle/tube/spray can/whatever.
How do you know if it's old when you buy it from the store? It's like buying a steak -- check the expiration date (really; they're on there). If you're past the date, pass on that bottle. Expired sunscreen won't hurt you, but it has broken down and it's just not effective.
Ideally, you'll use the bottle up the same year you buy it, chuck the empty container, make it through the winter, and then buy a new one.
So glop it on, wear a hat, and don't live under the sun. You'll keep some oncologist from buying a yacht, and you'll be a lot healthier.