You don't need to.
There are two factors at work here:
1 - the aerial. Where I used to work we have been experimenting with this as a test (payment systems design). About a meter in diameter gets you clean reads from up to two meter, provided you address factor 2:
2 - sensitivity of the receiver.
When this whole RFID lark started there were plenty of reports of people queuing who ended up paying for someone already at the cash point. That was basically solved by using far less sensitive receivers. Get a decent receiver stage and you'll enlarge the distance, but best in combination with a good aerial because you need to supply the power for the circuit to work first. That said, making it a terrible receiver also serves another purpose - it allows the payer to pick the card they want to use instead of randomly pick one from whatever it picks up.
The reason you're not robbed blind is not the tech itself, it's the many checks to get an account that accepts RFID payments (processing can distinguish between RFID and a 'regular card-in-slot plus PIN' payment). Basically there is a process plaster over rather too permissive tech :).