How to Avoid Being Too Partisan

1. Pick a couple of issues you feel strongly about, one on each side. When you're too close to being a partisan, remind yourself “There are the people who are wrong about X.” Picking two issues will prevent you from being partisan on the other side, in case you switch. It might even be helpful to pick four issues, two where each side is wrong about the facts and two where it's wrong about the morals.
2. Make sure you read something on the Other Side regularly, preferably from people you have something in common with.