I used to have some methods for gaining my tolerance back.

Of course, moving on to harder drugs works, but I would never recommend that.

Now, when I feel my tolerance getting bad I just get completely different weed (ie: going from mostly indica strains to mostly sativa strains) I feel like I build up a tolerance for strains, rather than weed in general. So I've been trying to switch it up every now and then, and my tolerance is still somewhat good.