There's 2 ways to do it with AA....you can book a one way and then when dates come into computer for the return, you book a 2nd one way home (this is how Ngaire and Ken do it I think) OR
you can do the way I do and book the entire RT even though the return date is not correct and then when the dates open up you can change to the "correct" date. ...either should work since AA allows f.f. one way tickets (at 1/2 of the RT miles)and doesn't charge a re-write fee for a change in dates like stupid Delta does.