Since it's not yet possible to increase the environment light with the path tracer (you can do it of course, but it has no effect), and Matt wasn't very excited about such a feature, since it's not physically correct, there's no other way but using hacks. Personally I don't like the idea of using no shadow suns, since it produces a very flat lighting to my taste, and encreasing the exposure would be the most correct way, but it makes everything brighter.
So my suggestion is using a very large white selfilluminated plane set to invisible and with "cast shadows" turned off above the scene. You can control the effect by modifying the luminosity strength. I don't know if this works for a dense forest, but at least it takes some shadowing into account, which makes it look a bit more real than adding lightsources without shadows. Maybe in combination with a reduced opacity of the leaves.