As mentioned above, if you are pretty sure at booking you won’t use the return, book it for a date as far out as possible while still getting the fare you want. Then, if plans change, you can call in and change the return for the fare difference.

Also consider booking the return on a date that will likely be very busy (day before Thanksgiving or a long weekend) where delays may cause a cancellation or misconnect at which point you may be eligible for a refund.