What does it sound like when you try to start it? Fast Click - usually battery, Silence with one tiny click - Starter, If its starting to die while ur driving then i would say its the alternator. Also check the ground wires going from ur (-) on the battery the engine block and chassis. The check the ones on the (+) side going to the engine block, the body, and to the alternator. If these connections are corroded with rust, dirt, etc. this might be the problem. Also check the connections on ur battery terminals. Then check all of ur plug wires and the coil connections.
That should just about do it!
oh and go get ur battery, starter and alternator checked. Process of elimination ;)