With 'full service' airlines, such as BA, it's possible to purchase flight tickets which permit changes. You simply need to select the 'Flexible', rather than the 'Lowest fare', option on their websites. However, you pay a massive premium for the privilege. For example BA will fly you London to Paris for �88 return if you select the 'Lowest fare' option. Travelling in the same seats, with the same level of on-board service but with 'Flexible' tickets, will cost you �402.
On that particular route, you can purchase single 'lowest fare' tickets for half the price of a return ticket, so it would make sense to buy two singles. (Even if the fare back from Paris to London had risen by the time you booked it, it's unlikely that the aggregate would be anywhere near the flexible fare). However, with some full service airlines and on some routes, a single 'lowest' fare is often quoted at a price far higher than the lowest return fare! The only way to find out what will work best for you is to visit the airline's website and try a few dates to see what's offered.
With budget airlines, such as Ryanair, there are no return fares. If you book a return trip, you're actually booking two singles. In some cases (if you require flexibility), it makes sense to buy one outbound ticket and several return tickets.
e.g. if you know that you will want to return on one of two possible dates and each of those dates currently has a fare of 99p + taxes, it might make sense to buy tickets for both of those dates. Otherwise, if you leave your change of plans to the last minute, you might find that the fare for the flight you require is now �149 + taxes.
Unfortunately, there's no simple answer to your question. (You need to visit the relevant website and try out a few options). However, I hope that this post will provide you with a few starting points.
Chris