Haze comes in different types and colors, and that means that there are a lot of things people do to try to fix it. I asked the same question once at a workshop and there were as many "always works" technique answers as there were people in the class.
Nik Multimedia has a couple of filters in their set that help some - one is a skylight filter. The other is their "contrast only" tool, which is a fairly sophisticated and controllable follow on to doing as much as you can with the clarity slider. If you can take a peek at the book Window Seat, by Julianne Kost, she has some techniques that work great on high altitude haze that includes working with the blacks slider, curves, and saturation. (Try a curves layer set to "luminance" - it won't build up saturation that you don't want.) I find that with urban smoggy haze I often need to use the white balance eyedroppers in addition to curves. Ocean haze (if the air is otherwise clean) is easier to adjust for if you're close in, but I've found that anything much out in the telephoto range, you lose a lot of detail besides losing saturation. You can kind of fix one, but not the other.
Unfortunately, every time I think I have it figured out I run into a new form of haze that my tricks don't work on. Today we have an inversion layer in my area, lots of warm air near the ground, cold above, and there's a thick haze over everything. I just pulled up a couple images and discovered that clarity, curves, saturation, blacks slider - none of them are working too well.
