The nice thing about thumb position is that it's exactly the same for us as it is for cellists, so we can use the same exact technique that they do and get the same results. There's a very good video of Jeff Bradetich online that showcases his impeccable thumb position technique and the divisions of whole steps and half steps here: Invalid Link Removed Everything he says is spot on and as you can see from his playing it works.
In thumb position, you'll always want your thumb pressed down a half step or a whole step away from your first finger (very rarely a minor third which I've used playing the Koussevitzky concerto to avoid a shift during a slur). The thumb will give your hand stability and aid in vibrato. For all shifts in thumb position you'll want to move your thumb along with the shifting finger the same way you shift in lower positions with first finger to accurately measure the shift and make sure you hand is as secure as possible. After that, there are the different combinations of whole steps and half steps between each finger. The most common thumb position configurations that I use are a chromatic "closed position" of all half steps, then a whole step between T and 1 with half steps between 1 and 2 and 2 and 3 and then a whole step between T and 1 with a whole step between 1 and 2 and a half step between 2 and 3. Sometimes it's needed to have a whole step between all fingers in a fast passage where you want to avoid shifting by moving the thumb. As the positions get larger, you'll notice that it's not very easy to keep all fingers down where the notes are and that's perfectly fine. As you play up or down the thumb should stay in the same place as a reference point and the other fingers down, but they can move towards the note that you're trying to get to a little and when coming away from that note go back into place. It's a very accurate, efficient way to use your left hand. And of course if you need to bring out a note one can move the fingers into a more condensed position to help with vibrato. I don't recommend the way Karr does it by bunching fingers on top of one another, but if, for example, you're coving a perfect fourth from G to C at the second octave with T to 3, you could always move the hand into a smaller position instead of keeping a larger one while still avoiding a shift.
One very good book on thumb position is Franco Petracchi's Simplified Higher Technique book published by Yorke. It's a little pricey and some of the drills are overkill to start on, but it's worth looking at.
Collapsing of the first finger normally happens if you don't play enough on the tip of the finger as the first finger needs to be at more of an angle to preventing collapsing than the second and third which are more pad and less tip. This is because of the angle at which the hand comes at the fingerboard combined with the necessary spacing of the fingers. One thing that might be holding you back is having long nails. I make sure that there are no whites at the end of my fingernails so that I can as easily as possible in thumb position. It's difficult at first, but if you keep at it you'll have curved fingers in no time and have an easier time shifting and playing faster. If your third finger is collapsing don't give it too much attention. The angle at which it comes at makes it preferable to play more on the pad to get an even vibrato with the other fingers.
Gee wiz, I'm scared now...better buy your books ASAP so I don't fall behind...
