I believe few amateur astronomers see UFO's because you are looking at such a small slice of the sky at any given time and if they cross your field of view it will be very very brief.

I used to have a telescope and did some star gazing, and never did I see anything in the way of UFO's. Nor actually with the naked eye.

But last August I took a ferry from Edmonds to Kingston Washington and from the boat I took some photos.

I didn't realize it at the time but later while processing the photos I discovered I had captured one in one of the photos.

The UFO was so close to the color and brightness of the sky that it went unnoticed until I used PaintShop Pro's "clarify" function which increases the contrast on relatively low contrast objects such as a blue sky.

Take a look on my ET/UFO blog:

http://www.eskimo.com/~nanook/et/

You will see part of the dock (I took this picture just as we were leaving the dock), and in the upper left you can see the UFO.

You will note the contrast is exaggerated, it would not be very visible without that enhancement.