Under the umbrella of fly the plane first is collision avoidance, be it with terrain, obstacles, or another aircraft. It is critical to pay attention to where you are going; especially in VFR flight at or near an airport, it is vital to keep your head on a swivel to see and avoid other air traffic.
Simply moving your head rapidly around your field of vision is not good enough. Turn from your computer monitor 90 degrees to your right. You can focus on either point but you're likely to miss a lot of the details in between. In an airplane, that could mean missing air traffic in your vicinity. Current recommendations are to change your viewpoint slowly, scanning the sky around you in small sections and stopping to really look for a few seconds.
Helping other traffic to see and avoid you is beneficial as well. I was taught long ago that navigation lights and strobe lights should always be on when the airplane is moving. If you are in congested airspace, turn on a landing or taxi light to increase your visibility. If you are operating at an untowered field with a Common Traffic Advisory Frequency (CTAF) or a Unicom, monitor it and pay attention to where people are relative to your position. Even in 2009 some people fly without radios; unless you're really out in the bush, don't do that.
AOPA has a few scary stories about in-flight collisions involving small aircraft archived on their website. I have never experienced one of these events, but I have experienced a close call with another airplane. Back when I was first learning to fly in Maine, I was just outside the traffic pattern area of KLEW, Auburn/Lewiston Municipal Airport. We were roughly over the Auburn/Poland border and we were heading inbound towards the airport. I was looking forward and to the left when out of nowhere a flash of yellow screamed across my field of vision, maybe 500 feet beneath us but close enough that it startled me. I caught a glimpse of two wings and then I lost sight of the other plane. It scared the heck out of me, but it was a valuable lesson that even in the relatively un-congested airspace of central Maine there is other air traffic and complacency is bad.
The other aircraft may never have even seen us, and he was not making any advisories on the Auburn radio frequency. It blows me away that some people still fly without radios when a handheld can be bought for a measly $400.00...but that's a whole different post.