I find that rotating sprays works well for mine. The stuff in the purple bottle, Bug Block, worked like magic for my boys at first, but by the time the bottle was empty wasn't as effective. I just opened a new bottle of Bronco Gold, and it's a lot more effective. I think my bugs develop immunities or something.
Other sprays I like are Tri-Tec 14 and UltraShield, but those are pricier.
I also buy a sunscreen/fly repellent gel at a local salvage-freight place. The brand is American Horseman, but I've never found it anywhere other than the salvage-freight store. Keeps Scout's white nose bug and sunburn free all summer, with none of the drop in effect I get with most repellents.