Something with lots of DEET is the most proven way, though there are a lot of possible issues with that, and if you're going off trail a lot you probably can't beat the "deep woods" brands that are something like 25%.
An alternative is Avon Skin So Soft. Theres a good bet you'll hate the smell, and it has shown to be less effective than DEET but seems to be regarded as completely safe. But, to me I feel like I've always had better results for repelling gnats/flys using it over DEET based sprays.
An alternative is Avon Skin So Soft. Theres a good bet you'll hate the smell, and it has shown to be less effective than DEET but seems to be regarded as completely safe. But, to me I feel like I've always had better results for repelling gnats/flys using it over DEET based sprays.