Unless you know for sure the day or even just the week of your return, book a one-way.
Airlines are increasing prices daily it would seem. Even using budget sites like Kayak.com may not yield a favorable price for your next trip. Although there are plenty of tips as to which days to fly that may be more affordable, the real trick is in the return flight. If you’re studying abroad for example and are still unsure if you will travel some after your semester ends, buy a one-way. That way, as the time approaches, you can search for prices with enough booking time and not feel the time constraints of an already purchased ticket. Plus, you eliminate the risk of paying a fee to change your flight. You still have to plan on enough time between booking and departure, but that becomes easier as the desired date of departure becomes clearer. Buying one way may be especially convenient when flying domestic.