Studies have shown that pilots don't (can't) detect most traffic visually:
Air Accidents Investigation: 5/2010 G-BYXR and G-CKHT[/url]
http://humansystems.arc.nasa.gov/fli...vin_ISAP05.pdf
www.atsb.gov.au/media/32918/limit_see_avoid.pdf
etc...
An exception is in the circuit, or other situations where you know where the conflicting traffic is most likely to be. For example flying down the Luton/Stanstead corridor I would concentrate my search down the corridor rather than to the sides, because you're more likely to have VFR traffic flying along the corridor rather than flying into you from controlled airspace.
The real answer though, is that mid-air collisions en-route are very rare, and on the principle that we should focus our attention on those things that we can change and that are significant, there are better things to worry about. Not that I'm saying you shouldn't look out of the window.