to aviod mites, keep it clean (remove discarded food asap). to avoid fungus, a dry spell for a few hours would be great. also, peat moss, due to its acidic nature, can greatly reduce fungus. best way in my opinion is to the substrate moist--misting is just too much work for me.
Humidity, in general, is over emphasized in T keeping. That said, misting doesn't do a thing really to address it since it evaporates in a matter of minutes to hours and, unless you're some sort of masochist that mists your tanks 3X a day, won't do a thing towards maintaining a humid environment.
Keeping the substrate moist is a good way of providing a good local rise in humidity at the ground level, plus the T is free to burrow or climb a bit as it sees fit to find where it wants to be.
Still, in my opinion, if you really want to maintain a high ambient humidity the only thing that will really do it is to get a room humidifier. Too many species like the substrate dry, and too many problems are created by restricting air flow to the point that it maintains a higher humidity.
Still, I don't worry about it. Ambient RH never climbs above 40% for any of my Ts. Moister species get moistened substrate, everybody else just gets a water bowl. If I ever find myself in the NM desert, I'll just get a room humidifier instead of messing around like so many people like to do.
2) I have a vaporizer and a humidifier going at all times in my tarantula room. I have a very dry apartment so I need this.
That is usually sufficient for most of my species... but if thats not enough then I have 2 more cards in my sleeve that I use...
3) I lay a damp tea towel across the top of the cage where the vent holes are. This still allows ait to pass while adding extra humidity.
4) Under the water dish you can put an under-tank heat pad. This warms the water and makes it evaoporate quicker. Just be careful not to let the water get hot (Have some insulation between the dish and the bottom of the tank such as substrate)