There used to be a number of work-arounds that were available to agents; some of which still work (though I am not going to reveal all our moves) and some which have had to fall by the wayside. Buying return tickets for one-way journeys is still, however, a useful thing to do.
For example, a single ticket from London to Washington, DC would cost £1,199.75 and yet a return ticket could be bought for £538.57 (on the particular dates used in my example). It does depend on where you are going, of course. A single ticket from London to Vienna on Austrian Airlines will cost you £149.30 and a return £146.40 - only a very small difference. BUT if you knew you were coming back at some stage, then, given the price difference, it would be as wise to "take your best shot" at your return date. At the lowest rate, a single ticket from Vienna to London may be had for £73.00; taking even a wild stab at a proposed return date makes sense.
The next most important point to note is the type of fare being purchased. In the Vienna example mentioned above, the fare does not allow any changes at any price, so, if you get your "best shot" wrong, then the return part of the ticket is lost. What one can do, is to look at the "classic" fares. Here, a one way fare would cost £169.30 and a return, £177.40. Now, at this fare, changes can be made at a cost of 70 Euros PLUS any fare uplift. This, then leaves a dilemma: If you do need to change, then there is a good chance that it will be quite close to the departure date. Instead of having to pay £72 for a one way ticket from Vienna to London, chances are that the one way fare would be close to £230.
Of course, by the time one has taken the fare uplift into account and paid the change fee, the actual fare cost may be about the same. What one has to bear in mind, though, is the tax. If you buy a new ticket, you have to pay all applicable taxes "and charges" again, if you change your current ticket, then the applicable taxes have already been paid.