If you need to keep your sunglasses from falling off your head, just grab a few rubber bands.
Step 1: What You Need
I was heading out on a bachelor trip where we were going whitewater rafting so I needed some way of keeping sunglasses from falling into the water. I didn't want to buy a Croakies-style strap I'd use only once and I didn't care how I'd look.
1 cheap pair of sunglasses. Or nice ones, but mine always break so I get cheap ones.
4-5 rubber bands. My mail comes banded with these every day so I have a bunch.
Step 2: Linking the Chain
Now you chain the rubber bands together. To do this, stick one rubber band inside the other and, reaching through that one, grab the other and pull it through. Just look at the pictures.
Repeat until all of the rubber bands are in a chain.
Step 3: Attaching to the Arms
Attaching the rubber bands to the arms is tricky to explain so check out the image below.
First, you'll be folding the rubber band back down and reaching through to grab the rest of it.
Second, thread the arm through the rubber band so that the band is looped to itself when you let go with your hands.
Third, pull on the band and get it tight. This can take quite a bit of finessing since the rubber grips the arm something fierce. Keep trying, have faith, and keep pulling without breaking.
Step 4: Finish Up and Look Classy
Before attaching to the other arm, try the glasses on to see if you need to add or remove a rubber band fo a better fit. Remember to have a good amount of slack. These are meant for keeping the glasses from getting lost, not keeping them ultra-tight against your head. When everything's good, just loop it around.
My glasses held on fine for the two-hour ride, but since I didn't get thrown over I didn't get to stress test them. It was my first rafting trip and we all had a blast.