Have you tried using templates to scribe circles and so on? They really make it easier than trying to do them freehand. You can find a variety of them with different shapes in any office supply or craft store. Also, aftermarket manufacturer Verlinden makes flexible metal templates in 1/32, 1/48, and 1/72 scales that assist in scribing a variety of shapes that are specific to aircraft. Eduard even makes templates for specific kits. Sprue Bros. carries both Verlinden and Eduard stuff.
Whether using a template or not, I've found a needle chucked in a pin vise the easiest to control for doing circles, since it has a finer tip than most scribing tools. For me the trick is to make a very light first pass, then applying increasingly greater pressure to deepen the scribed line. I then use a conventional scribing tool if I want to deepen the line further. Like anything else, it takes practice.
Another scriber worth mentioning is the one made by UMM. It's primarily intended for scribing, but it's design allows for a variety of tasks:
http://umm-usa.com/onlinestore/popup_image.php?pID=474&osCsid=a704e977de035bd00268fdd63fb3beeb
Great tool.