If you are wearing makeup in the summer heat and it is melting... you probably don't just need to touch up the makeup but also your sunscreen.
Powders with SPF are great, but it can get cakey.
Since last year I have been using Korean sunscreen sprays. It's so easy to use, I really think they invented this for lazy people.
Here is the one I currently use and it's completely clear (no white flakes). It is like spraying water on your face.