Check for a round trip ticket that's good for greater than 6 months. Some are good for up to a year. An agent could help you with this. The extra cost is still probably less than two 1-way tickets.
I say 'probably' because I flew LAX-NRT once on a one way that was actually LA to Seoul. I just got off in Narita and didn't get back on. It was cheaper than a RT at the time and I was given a tourist visa at immigration.
Check on the amount of time you must be out of Japan to return on a tourist visa. 2 weeks comes to mind but that's just what I remember w/o looking anything up.