I have been using acid free photo corners, but occasionally get an unpleasant amount of buckle in the print. I think that the key is to have enough overlap so that the print is held down well. If you are doing the 1/4" overlap that standard sized pre cut boards come with (7 1/2 X 9 1/2 inch opening for an 8X10 photo) than you will see some nasty buckling and I don't know of a good way to hold it down other than dry mounting. The photos where I have an inch on each side hold down pretty flat.
I haven't been hinging the top mat to the bottom, but I am going to start doing that. The risk is that you will have some movement in the frame and that will cause your image and over mat to no longer line up.
I have been buying metal Nielsen frame sections from American Frame. I am getting the ones called "Canvas" which are super cheap and hold a very thick collection of stuff if you like to do double over mats or float your over mat or something of the sort. 16X20 sections are $12 and they ship much quicker than Light Impressions. I have not found this particular Nielsen section nearly as cheap, not sure why since their prices on the others are very similar to everyone else's. Oh, and they include the hardware for the price of the sections, Light Impressions charges extra.