While a good lookout appears to be required, I don't think a good lookout is anywhere near a solution to the (already generally miniscule) risk of a mid-air.
If an aircraft is on a TRUE collision course then it isn't likely you will spot it. If it is head-on (recip track) then you are very unlikely to ever see it.
So a good lookout will make you see more planes, and will make you take avoiding action more often, but I don't think those planes would have hit you anyway.
Universal use of transponders and proper (currently £15k+ fitted cost) traffic detection systems are the only way to detect traffic that is on a genuine collision course.