There are a lot of posts on this board on anticipation and split-stepping, so I won't repeat all that stuff - except to say that they are important and need to be practiced. You can search for them quite easily.
There are a couple of things I've learned the hard way, and I invite comments on them. Both are techniques to "slow down the ball" - I am using quotes because, of course, the ball doesn't really slow down, it's just one's perception.
First - it's easy to keep watching the server's antics, and forget to focus on the ball as it gets close to you, especially on lightning fast first serves. Make an active effort to shift your focus immediately after the hit to the area between the bounce of the ball and where you are positioned. If you are not quick enough, your focus will still be on the opponent's side of the court, and the ball will be on you in a flash as you are flailing at it. You'll see that once you shift your focus to the segment after the bounce, the ball appears to be much slower and controllable.
Second - and this one is probably more controversial - you have to crouch a little to get a better reading of the ball. My formula is to crouch just enough so that my eyes are approximately at the level of the ball as it gets closer to me. For example, if I know the ball is going to be between waist and chest height after the bounce (standing straight), I will crouch enough so my head is around that level. It doesn't have to be perfect, so don't get carried away! My reasoning is this - the eyes have to move much less to focus on the ball when one is directly behind it, than when one is watching it from the side. This works quite well for me, but I haven't come across this anywhere. A lot of pros seem to do it, though.