Something that I saw other pbm companies do was giving a discount
when a person credited their account with a large amount at
once. I
guess sort of like a discount for buying in bulk type.
For example,
if you credit your account with XXX quids then you'll get an
additional X% added to your account as a bonus or X number of free
turns ?Perhaps another idea might be to pay in advance for a game ?
(etc)
Come back, Chris Harvey.... 
Gavin