Booking one ways with different carriers worked out great for us - someone else suggested it on the board (wish I could recall who and give credit) - I figure we saved about $300 each on the round trip booking that way. At that time, each airline had one direction that was cheap and the other that was pretty expense. Made the round-trip price about the same on both airlines, but since one was cheap going down and the other cheap coming back, it was great deal for us.

I normally used Expedia and Fly.com to check on prices in general and to see what airlines/routes are available to my destination (its almost never a direct flight for us), but once we had picked our dates, I started checking with the 2 contender airlines directly, looking at round trips and one ways. When we got into a realistic window for buying the tickets, I was checking the flights a couple of times a week when I thought of it and had a moment to spare. I had a price in mind and when the price dropped below that threshold, I pushed the button. That was mid-August for our trip the end of November.

I also really like using http://matrix.itasoftware.com/ for researching flights and prices.