I started out by getting the edges in place, so I had a frame of reference. After that, I started working on one of the short sides, as those pieces were pretty easy to recognize. Once that was done, it degenerated into 'hey, I think that piece goes around here' and making little piles until there were enough in an area to actually start matching. Or just making matches as I picked through the pieces even if I had no idea where the match belonged in the larger puzzle... There was a half-hearted effort at sorting by color/pattern, but that didn't really start being helpful until I had made progress on some of the smaller sections. Now I have several flat pieces of cardboard that I'm using as platforms for particular colors or patterns. (Particularly the 'borders' and 'arches' - there's quite a bit of those. Having them concentrated in one area makes it easier to spot the matches.)