How do you mark your 'in' point with the keyboard?

I know you can mark the first frame of your selection by hitting the mouse button but I find I press the spacebar to start previewing the source footage and stop the playhead at just the right point where my 'in' should be. Now as soon as I move the mouse the playhead jumps to a different part of the clip, this results in me losing my 'in' point.

What I want to do is hit the spacebar to stop at just the right point, hit a key to mark my selection, hit spacebar to resume playing and then finally hit shift-A to mark my out. Any ideas?

Thanks,
Dave