Buy a whole bunch of frames. Buy more than you need, you can always take some back. Buy a variety of frames, but mostly 4x6 and 5x7. Get a few 8x10s also. Try to stay away from more than two of the same frame in the same size, but whatever. It might also look good with all frames EXACTLY the same. I grabbed a couple composite frames as well- three or four photos matted and mounted in the same frame, but chose not to use them. It might work well in your setting, esp. if there is a common theme in a few pictures.
Step 2: Arrange your frames
I started out putting them on an old sheet (see picture), but quickly became smart. The key is laying them out on Christmas wrapping paper. Measure your wall and spread out enough paper on the floor to cover the wall area in question. This will be especially useful later. You'll see. Anyhow, lay the frames out and arrange them in portrait and landscape configuration however you'd like. I figured the upper and lower boundaries and line space between frames should be close throughout but not exact. You might like more precision. This step is key, because it is very difficult to change the frame order and placement after this. Also, if you've gotten all identical frames, you'll need to insure your lines are tight and even.
Step 3: Trace your frames and mark your holes
Now that your frames are sitting on wrapping paper on your floor, you should trace around them (or at least the corners) to mark where each will go. A felt tip pen works well for this. Then remove the frame and number the newly empty paper square and the back of the frame. Mark where the nail hole(s) need to be for that frame. If you have any frames hung with a wire, just mark the frame location. You'll probably have to fine tune the exact placement later once the other frames are on the wall.