Funny, the technique I was taught is the opposite - to concentrate on parts of the sky and move your eyes around this slice (say, 20%) in a sawtooth motion, then move on to the next slice, etc.
In reality, my guess is that 99% of mid-airs are avoided by the Big Sky theory and only 1% by the employment of MkI eyeball
In other words - Özcan, you are not alone. As others have said, the most difficult object to spot is the one that's on a collision course with you, i.e. static in your windscreen.
My own technique, if ATC call a conflicting traffic and I simply cannot see it, is to at least make myself as visible as possible: turn on strobes, perhaps landing light and rock the wings.