I'm surprised you got back to the airfield with your pants still clean saab after the 30ft incident!
In my 100 hrs I have not had any close calls like the above. Although I was flying north of Bournemouth one Sunday a few weeks ago and had to turn to avoid 3 aircraft in 40 mins. All at the same level, closest was about 200m horozontally....although we both had seen each other.
It's very true that it's difficult to see aircraft on an collision track......I also know that alot of collisions happen because one or both aircraft were in each others blind spots.
I always try to keep a good lookout, but theres alot to be said for climbing higher.......I find that even at 4000ft theres much less traffic.
Also I usually talk to the nearest radar unit if I can.......they can be a good help at times. If I'm flying from A to B in a straight line I'll ask for a RIS......Although often it's easier said than done getting a RIS!!