there are a few options ;-)
generally:
a) have a table reserved up in the vip
b) know someone who has a table reserved up in the vip
c) wear long trousers, walk up to the vip entrance, shake the bouncers hand with a 10€ note (or similar) and walk upstairs. works for most parties and gets you upstairs if you look alright in the bouncers opinion. doesn't get you in the tables area in the vip though.
d) buy a vip tkt online here!!! (for cream for example)
e) for the lesser famous parties at amnesia you might get upstairs without paying extra or having a special ticket. always wear long trousers though and a pretty girl on your sidealways helps as well
more options anyone? all roads lead to rome anyway... ;-)