Good thred....great topic.
First of all, let me express disagreement with all those who assume that slip is not a spacialised position. May be the BD think tank also assumes the same
I have been a slip fielder myself and I know what it takes to take the success rate upto 99.99% (no one can be perfect).
Slip poses various challenges that other positions dont. I have seen very good fielders who have dropped catches at slip. For India, Srinath, with fast legs and a very strong arm, was an excellent sweeper. But was a very bad slip fielder. On the contrary, Laxman had always been a bad fielder in all other positions. But did an excellent job at the slip.
The challenges at slip:
# (from what I recollect from a Jhonty Rhodes interview)- For all close-in positions includins slip, the fielder must expect the batsman to play a shot in his direction everytime. - it applies to slip as well. The fielder must pay full attention and expect an edge to fly in his direction each time.
# Be careful about the spin on the ball: An edge at times induces a heavy spin opn the ball that makes it very difficult to grip it. The ball tends to bounce out even from the softer part of the palms. Watch out for the spin and practice catching heavily spinning balls at a good pace.
#Be careful about the swing: This is rare but happerns. Sometimes the ball tends to swing on its way to the fielder. It tends to move away from the fielder creating a curve on his face. You must ensure that you cover the swing, time the dive right, dive the right distance and affect the right pull after the catch. Otherwise it will spill easily.
# Diving: Depends on how many fielders are there on each side and whats the distance between you and them. You have to judge whether a catch be suited better for the other slip fielder. On so many occassions we see two slip fielders diving for the same catch. For the first slip, its important to memeber that the wicket keeper can dive better, a longer way and has gloves.
#Style: The fielder gets a fraction of a second to take a catch. If the catch is coming at a awkward height, it becomes difficult to decide whether to take it in the australian style (fingers pointing upwards) or the traditional style (fingers pointing downwards). When the catch is at an awckward height, its advisable to adjust quickly to one of these two styles coz the style inbetween the Australian and the traditional styles will make the ball hit the hard part of the palms and in 5 out of 10 cases, the ball will pop out.
#Keep your hands free: Like SM Hasan pointed out earlier, dont rest the hands on knees. That does two things: one, it keeps your hands busy and two, you end up putting your weight on your knees....takes time to get up and dive.
#Stance: Make sure what stance suits you best. I have seen Sachin Tendulkar use different stances in different matches. Sometimes stands stretching his legs away from each other and some times upright. He probably does that depending on whether a side wise dive is more necessary or an upward jump will be more necessary (depending on whether the ball is swnging more or buoncing more and at what speed the bowler is bowling)