From a controller's point of view: take an appropriate service and make sure you understand what the terms of that service are. I'm frequently amazed by pilots who just reply "roger" to everything and I'm certain they don't really take in what I'm saying to them.
Once I passed traffic information to an aircraft outside controlled airspace along these lines (good VMC):
ATC: "G-CD unknown traffic 12 o'clock, 10 miles, opposite direction, same level, report visual"
PILOT: "Roger, G-CD."
ATC: "G-CD previously mentioned unknown traffic now 12 o'clock, 5 miles, opposite direction, same level. Do you have it in sight?"
PILOT: "Negative, G-CD."
ATC: "G-CD previously mentioned unknown traffic now 12 o'clock, 3 miles, opposite direction, same level. Do you have it in sight?"
PILOT: "Negative, G-CD."
ATC: "G-CD previously mentioned unknown traffic now 12 o'clock, 1 miles, opposite direction, same level. Do you have it in sight?"
PILOT: "Negative, G-CD."
PILOT: "We missed him, G-CD."
There was plenty of time to turn or change level as I had given plenty of warning of the unknown traffic, but the pilot seemed intent on getting as close to it as possible. This has happened to me more than once.
I suppose the point of my post is that although keeping your eyes out of the window is the most important, an ATC service can be a big help for avoiding collisions, infringements etc but it is important to remember who is responsible for what under the service you are getting. Also that being in receipt of an ATC service does not make you immune from conflicts and you still have to put some effort in yourself.
I'd be interested to hear what the private flying fraternity thinks of the new services outside controlled airspace. I've certainly noticed from my end a lot of people who used to take a FIS are now taking Traffic or Deconfliction services, which I think is a step towards safer skies.