Why would you use a potentiometer in the first place? I think you're confused.
I'm no expert but I'm guessing what you're looking for is an H-Bridge: H-bridge - Wikipedia
you may want to post in the FAQ section, as this is where people exhibit projects they made.
As skain says, the best way will be with an H-bridge
I think something like this is what you are looking for:
For future refrence though, this is the wrong section of the forum for questions.
Hey, I just had to point out, with the Schematic on that page for the L293D, the two power supplies on the right should be on the 5v of the microcontroller.
If you're going to try either of the two, I'd go for the second link supplied.. Tom Igoes the man! The 754410 he's using in the tutorial is pin compatible with the L293D, but can do 500 ma more per channel!