At the beginning of the summer, we went on vacation somewhere about 20° hotter than it was where we live.
Right before we left, I read an article that talked about how heat can break down sunscreens and make them ineffective. I thought a week in a hot, hot car would not be the best for our brand-new, beginning-of-summer tubes of sunscreen.
I put the sunscreen in an insulated lunchbox and kept an ice pack in the top when we were away from home. Not only has the sunscreen remained effective through the summer (we have had NO sunburns this summer, hooray!), it has been much nicer to put on. I hate putting on warm, oily, slimy sunscreen.
As a bonus, the sunscreen is always corralled in one place now, and it's easy to grab if we want to take it with us.
In any case, I just wanted to share this tip in case it comes in handy for anyone else.