If you're flying somewhere and staying there, or aren't sure when you're coming back, buying one-way airfare seems like a no-brainer. After all, you can always purchase a return ticket later. However, you need to take into consideration the growing popularity of discount airlines and airfares, increased competition between discount lines and legacy carriers, and the surprising truth that round-trip airfare is frequently cheaper than a one-way ticket. If you decide to purchase one-way airfare, bear in mind the potential drawbacks.

Your Destination Matters

If you're purchasing a ticket to a domestic destination and do not need a return, a one-way ticket is almost always cheaper. U.S.-based discount airlines, such as Southwest Airlines and JetBlue, frequently offer one-way fares for under $100. The same goes for certain destinations in South America. For example, discount carrier Spirit Airlines sells tickets to Lima, Peru, on a per-leg basis. However, for travel to Europe or Asia, it's often cheaper to buy a round-trip ticket, especially if you prefer to fly with a major U.S. carrier.

Airlines Encourage Round-Trip

One-way flights are almost always significantly more expensive than round-trip fares, because airlines attempt to discourage passengers from booking for one way. Airlines, in general, dislike issuing one-way tickets because it disrupts flying schedules. Additionally, prices for one-way travel stay high because they're frequently purchased by business travelers on multi-leg trips, so corporate travel departments are accustomed to paying full price.

Beware Throwaway Ticketing

Throwaway ticketing is the practice of purchasing a round-trip ticket and throwing away the unneeded portion to get a one-way trip at a round-trip price. It's common among frequent flyers, but beware: Airline ticketing rules prohibit throwaway ticketing. If you fail to use the first portion and show up only for the return flight, the airline can charge hefty fees or even deny boarding. However, if you use the outgoing portion and don't show up for the return, the airline has no recourse.

Flexibility is Key

If you opt for a one-way ticket, being flexible will help keep the price down. A one-way flight leaving at 10 a.m, for example, will be more expensive than the first flight of the day -- often at 4 or 5 a.m. Consider flying during the week rather than the weekend; Tuesdays and Wednesdays are traditionally the cheapest days to fly. Search multiple travel sites and keep in mind that most travel sites do not post fares for discount carriers, so visit those sites individually.