The point that I cannot stress is enough is a fielder must have good anticipation. You must believe that every shot will be hit towards you and prepare accordingly. The importance of a catch or run-out can never be understated. A run-out chance could be the difference between winning and losing a match. Especially when the opposition is in a good position. Take for example the wicketkeeper. What happened if he didn't think about the next ball? You must think of yourself in the same way. Top quality teams usually have excellent fieldsmen who anticipate well and are always moving. Ricky Ponting, Paul Collingwood and Hamish Marshall are among those who you should model yourself on while fielding.
Just as important is to relax I between balls. Fielding therefore is a lot like batting. After you play your shot for example, "switching off" before the next ball keeps you fresh mentally. By this I mean thinking about something completely irrelevant while the ball is dead. In the field, whether you are at slip or on the boundary, it is vital to have periods where you focus on and off. I like to call it "switching on". As soon as the ball is considered "dead" (for example the wicketkeeper catches the ball and passes it to another fieldsman), this time is your "switching off time". Think about the television show last night. Play a song in your head. Whatever makes you relax or takes your mind off the game in progress. Then when you see the bowler at the top of his mark, begin your concentration and focus on the next ball. Steve Waugh called the degree of concentration when fielding "zones". You will be surprised how simple but effective this is to improve your enjoyment and focus while fielding. It is also necessary to last a whole day in the field!