Stay very still, would you shake a camera when trying to take a photograph? Your eyes are tying to get the best image of the ball, keep them still.
Im going against the traditional here but you don't want soft hands, soft hands are weak and will most likely the ball will burst through at pace. If you watch and talk to the best slip catchers as I've been lucky enough to have had the chance too (Im talking Kallis, Waugh, Collingwood, Smith) they have strong hands and wrists with relaxed arms to absorb the impact.
With regards to feet position the knees are the most important part of the lower body, many of you will have seen the slip catches NZ dropped in the recent series. Mostly because their knees restricted the movement of the arms and upper body. Its different to keeping wicket, you get very little time to pick up the line of the ball before an edge, stand and react is the name of the game. Knees in or knees out, the choice is yours but make sure you can move your arms freely and rotate your hips to access the low catches outside your body width.
Long post I know, but if anyone has any questions feel free to ask and I will do my best to answer them