Can anyone give me any tips to try and counteract this problem.
My thumb after playing any string seems to fly out away from the strings almost in a clockwise circular movement. The result is that I seem to have to travel a long way to get to the next string to play with the thumb. I just can't seem to minimise the upward movement.
It's as difficult to describe as it is to rectify.
I remember when I first started my fingers were doing the flying away to. I had to slow down And pay attention to my hand. Even today I like to slow down for timing and accuracy, in other words Take it slow.
...this is as good a place as any to recommend Jack Hatfield's "Banjo Board" (hatfieldmusic.com/page6.html). It's an excellent practice and learning aid on which you can practice finger/thumb control just about anytime and anyplace. I've had one for some time now, and I use it a lot.
I am currently in process of training my thumb as well. In fact I posted a thread called "retarded thumb" (got chastised for using that word) and got some responses similar or the same as some here. There is obviously a little room for customization, as I look at many great players with slightly different hand positions and thumb movements. But my particular problem - not keeping the thumb extended and allowing it to move too close to the index finger - results in collisions and wrecks. Two members here - texasbanjo and Richard Dress - have been the most helpful for my particular issue. So take heed. I have slowed down, positioned my hand with adequate arching, and am trying a fairly perpendicular action to the strings with my index and middle, and also as perpendicular with the thumb. The thumb sees to like striing the strings toward my index. I suspect a little bit of that is tolerable. Funny there is so little written on this - it seems like it would be a fairly common issue. I also have determined that the incorrect movements are more common when you ask your thumb to do more than just pick the 5th string.
My hand is of normal anatomy far as I know, and I don't think I have any joint defects either. The thumb just seems like it wants to do what banjo teachers say not to!! Even Earl Scruggs demonstrates movement toward his index to a degree. But it's not severe. And I don't see him miss too many notes even at age 84!! His videos for watching this are good from Youtube. He attacks the strings pretty much perpendicular with the index and middle too as you will see. Tension is a killer I have learned, and so slowing down while I try to create some proper technique and get it into muscle memory keeps my tension low. Speed and tension for us rank amateurs seem to go hand in hand. Good luck to all you aspiring pickers.
I particularly have noticed the slight arch to the thumb (which most or all good players seem to have) and have worked on incorporating that into my hand and finger positions. It seems like if I maintain that slight arch, it keeps me out of trouble and the thumb remains extended instead of folding up against my index. That and keeping my thumb close to the strings are my two focus points right now and it seems to be helping. Maybe it will help someone else out there with similar challenges. I am staying away from songs and working just with roll exercises from books. Ross Nickerson's encyclopedia has a bunch. The forward/backward and the alternating thumb seem best for our thumb problems. Going slowly and trying to keep the "correctness".
Also watch a few J.D. Crow Videos and mimic what he does. It just takes time and patience, just like training a dog you keep working with them and they finally get it. Playing the Banjo is the same. Watch, Imitate and practice.