I generally go with this field for the first over of the innings:
Keeper (obviously) two slips, two gullies, point, cover, mid-off, mid-on, square leg (45).
If the bowler can't bowl to that then they are off, or we try to work out a plan.
I like to have this field because it gives the bowler and the batsman a message. It says to the bowler "BOWL OUTSIDE OFF STUMP!" (it takes some bowlers alot longer than others to realise this message). And it says to the batsman "hey, we have a swinging ball here, if you want to play accross the line and play with risk there are runs for you, but you make a mistake there a fielders there who will catch the ball." Of course you need the fielders to back you up, no point having them there if they can't catch or let runs through.
After a while I like to put another fielder on the legside, either the mid-off or one of the gullies so the bowlers can work more at the stumps, after they have got their rythum.
I generally don't like to have any more than three fielders on the leg side, but some time you have to keep in mind that the level of cricket I play at the batsman get frustrated if they aren't scoring runs, so sometime you have to strangle them and put four on the leg side.