there's a lot of variables. if you can't "stretch" or space your frame out with washers on the inside of the wheels, then a smaller tire is an option.
one of the problems is all tires are shaped different. you could buy a tire that says it's skinnier, but the tread pattern, sidewall design, or whatever might still stick out as far.
i had good results with Scwalbe Kojaks. they're a slick tire and are 26x2.0.
check out the review
BikeSomeWhere.com | Schwalbe | Tires - Mountain | Schwalbe Kojak Mountain Bicycle Tire - Wire Bead