Montana is a moderately large state. What part are you wanting to fish and what are you wanting to fish for? If you're going for western Montana I can give you some areas that have been good to me in the past. Eastern part of the state I have never fished.

Well if you get all the way over to the Flathead lake, troll at a moderate depth around wild horse island. There are some monster lake trout that patrol through the channels there. If river fishing is more your thing, the Clark Fork river which runs almost the same route as HWY 200 would be a good bet, I like the area between Noxon Reservoir and Thompson Falls. I've also had good luck at Hungry Horse Reservoir, fishing the inlet near the Murray Bay Campground. Take NFDR 38 (AKA East Side Road.) And all the way to the far west is Lake Koocanusa, (which extends into Canada) which empties into the Kootenai River. The kootenai will offer all different types of fishing as well. About 1/2 mile north of the swinging bridge/kootenai falls, is a great place for summer fly fishing if that is your thing.

If you're fishing the rivers and looking for some large trout, find a big deep hole. Especially if it has a decent back current. ;)

If you make it to the kootenai...
You can fish a short distance below the Kootenai dam...(there is an area marked to stay out of, because of water level changes, and I would recommend to follow the advice of the signs)... Has been known to produce some extremely large trout. (get some cheap fish(fresh) from the grocery store, chop it up and let it float down the river. the big guys just sit there gobbling up fish that come through the turbines. I think a few years back a guy caught a new record while fishing there, but I can't seem to find a pic of it. (although I know some of the local sports shops have pictures of it up)

Good luck!