This unbelievable simple hack will save you a lot of hassle. On my last trip to New York I bought two pairs of purple sunglasses that I absolutely adore! Problem was they were always slipping off my face and it drove me crazy! So I tried the following three solutions: 

First I bought a sharktank product called Nerdwax which comes in a chapstick sized container for $12.50 plus shipping. You're supposed to rub it on the nose pads and its supposed to create a tacky layer of friction between your classes and your skin. Well after many tries it did not work at all for me.

Second I bought a pair of anti-slip holders for $6 and they work perfectly! I just can't easily slip on and off my glasses as it's a whole process to wrap the holder around my ear. 

Then I just wrapped some ponytail holders around the ends of my glasses, and voila what a simple fix I could have done all along!