In the days before radio they HAD to lookout and were much more disciplined about (for example) circuit joining procedures. Ok there is a larger diversity of traffic around now but in many ways not a lot has changed if you are operating VFR.
Whilst not being a luddite I think the overuse of services such as Radar/FIS etc can lull pilots into a false sense of security. There are also more potential distractions such as the GPS, even changing frequency means head in the cockpit during which time another a/c can be filling the windscreen.
I know it's a generalisation (and I speak as one who has spent most of his flying career in civil aviation with a spell as a military flying instructor) but in the military there is much more emphasis on LOOKOUT since apart from anything else if you don't spot the enemy first he might shoot you down. So it's surprising to see that recent midairs have involved military operated a/c.
When teaching lookout on early details with students I tell them that lookout is an acquired skill and, at this stage, just because they cannot see an a/c doesn't mean there isn't one out there. Also don't get preoccupied with the one you can see - keep scanning!!
Finally, make sure the windscreen is clear - in summer a screen full of squashed flies can mask the target which might spoil your day!