Take the leadership in traffic separation, if you hear another aircraft on the CTAF, take the initiative and make contact with them, rather than just monitoring what they're doing, with the hope that you'll eyeball them, or that they'll avoid you. For all you know they're less experienced and they'll happy cruise along without a care about other traffic.
Work on nailing your radio calls to the AIP standard.
Log accurate ETE's and revising them enroute, plus groundspeed and fuel/mins remaining.
Fly as precise as you can.
Clean your windows.
Go on a long hourbuilding trip, at least a couple of days. I went away, alone for a week, flew 28 hours and learnt more in that trip than I ever had. It greatly raised what type of weather/conditions I could handle, went into some major airports and controlled airspace, as well as remote Australia and met some great people. Turn off the GPS and remember that "time map ground" does work and there are some simple rules of thumb to calculate everything you need enroute.
excellent advice tmpffisch and as was said above. Don't be afraid to use the radio! If you are unsure of their position/misheard their call and believe their track may conflict with yours ask them to clarify their track/height/quadrant/ETA etc.
One more thing, when inbound to a CTAF and you hear traffic call inbound at TOPD say around 50nm and you have the same/close ETA tell us then. You don't have to wait to you are 10nm. Tell us early so we can organise seperation or slow to facilitate circuit joining. Nothing more annoying than 3 aircraft inbound who have spoken to each other early, organised seperation and then one surprise aircraft calls 4 minutes out with an ETA smack bang in the middle of all the arrivals! Throws an unnecessary spanner in the works. Pilot to Pilot communication is not the devil.
Nav.