For long wear, the IRC's are very difficult to beat. The M5B that KDX1 refers to is the soft-terrain tire, but it still lasts really well. The intermediate IRC is the Vulcanduro VE-33 (rear) and VE-32 (front). IRC uses a really hard compound. Their sizes tend to run a little bit smaller than others, so you would probably step up one size.
A lot of it depends on your bike and what works best. I tried the Michelin M-12 on my 400EXC and absolutely hated it, especially in anything that even hinted at being slick or muddy. For my bike the S-12 or the Bridgestone MT-16 seems to work best so far. I'm gonna try the 756 this weekend and see how it works.
I used to run the IRC's on my XR 250 and they were terrific for that bike, but the VE-33 I have on my 400 now just doesn't seem to work well with the motor, and it also wore out prematurely. You can also get longer wear if you'll go up a size, but you must be careful about getting too much tire on a bike, where you end up with more tire than you have motor.