This is one of the measures of how good a turner really is - by how nearly perfectly semi-circular his/her beads and coves are. (Mine very rarely are by the way!)
Sorby do make a special tool for this, with an interchangaeble end for doing various sizes of coves and beads, but it is a bit expensive for what it is, and being a scraper, doesn't leave a great finish from what I have seen. This results in heavy sanding being required, which half the time ends up spoiling the profile. See here for details of the tool
Coves are turned using a spindle gouge with a rolling and sweeping motion. Beads can also be done with spinde gouge, but I find them easier to do using a 3/8" beading and parting tool, or even a small skew chisel, although watch out for the dreaded catch!
Have a look here for an enourmous amount of information, including a downloadable beginner's guide to turning.
just as Gary says... practise makes perfect.
For coves I use a Skewchy type gouge (or whatever suitable tool happens to be in the hand) but whatever you use it's always difficult to get both side to be a mirror image... one side will be easier than the other depending on whether u r right or left handed.
A deftly weilded skew turns a good bead but it's definitely easier with a beading / parting tool.
I've had a go with a few bits and pieces and I'm resonably happy with the result. I got what looked like decent results to ma anyway.
I've just received the books I ordered from Amazon as well as arranged a turning course locally. In two weeks I'm going to my first "radius" meeting too! Lets hope all these things can get me started eh?