Buy credits at 1¢ each... 100 for $1.00 (or use AmEx in the next couple of days for a double credit promotion)
Earn credits by participating in free tourneys. (iirc, some are 'win' and some are 'lottery')
Can't 'vampup your guy'... but you can acquire better clubs. Be aware though, that the best driver in the game only hits for 240 and it is not very forgiving.
Great comparison, Nivlac put together I think, somewhere on these forums.. use the search feature over there ----->
Hope this helps.