Rolling suitcases are great when you're running around airports, but if you don't feel like spending the money on buying one, Instructables user RonnyK has a relatively simple way to add detachable wheels to nearly any bag.
RonnyK's inexpensive process requires two Velcro straps and some wheels, which are added to the bottom of a duffel bag. This means you can easily remove them if you need to use the bag for something else. If you don't travel enough to justify the expense of a rolling suitcase, then this little trick might be all you need. Head over to Instructables for the full step-by-step directions.
Add Detachable Wheels to a Duffel Bag [Instructables]