I've been a long time lurker on this great site but I recently discovered something wonderful and after reading this thread, I had to join just to post this tip: I too had problems with circles but recently my husband was clearing out a kitchen cupboard and found an old Christmas present my Auntie gave me that has never been used. In the UK we call it a Lazy Susan and just in case you have no idea what they are I'll describe it! It's a round board (mine's glass), typically a chopping board that sits on another board and spins round, so whoever is on the other side of the board at a dinner table just has to spin it instead of reaching over to get whatever product they want. I am sure you can get them the world over!
With round cookies all I do is make sure they're placed in the centre, place my tip on the edge of the cookie (I place it 'west' which is where I used to start my outline going clockwise) and as I squeeze and lift, with my left hand I gently spin the lazy Susan. As long as I keep my icing hand still the icing line falls into the most perfect circle I normally only dreamt about!
Seriously, it works perfectly each and every time, instantly, and I've had not one dud circular outline since I've discovered this. I don't for a second believe I'm the 1st one to discover this, but I haven't seen anyone else mention it as a tip.