Look at your property's easement to determine who legally owns the driveway. Most cities keep easements in the land-records office. If both you and your neighbor have legal right to use the driveway, you need to split it equally. In some occasions, only one neighbor owns the driveway, and the other neighbor has no legal right to use or divide it.
Install a flagpole or post at the end of the driveway to divide it evenly. Each neighbor parks his car in front of the flagpole closest to his house.
Build a fence that runs down the middle of the driveway. Do not build a fence that blocks access to your neighbor's part of the driveway.
Paint a white or yellow line down the middle of the driveway to divide it. A painted line is not as intrusive or expensive as a fence.
Verbally agree with your neighbor to only use your half of the driveway. In the agreement, state that you will not block or obstruct his half. Agree to keep the driveway clean. A verbal agreement works well if you and your neighbor have a good relationship.