The game relies on a pre-determined amount of runs for a particular serving or strength of beer. In the real game of cricket (for the uninitiated) when the ball is hit over the fence on the full you get 6 runs, if the ball is hit 'to' the fence you get 4 runs.
To play beer cricket a full size can of beer is worth six runs and a smaller serve is worth four runs. Both of these serves should be of the same alcohol strength.
You can accumulate runs by drinking either of these drinks, scoring 6 or 4 runs at a time, However going to the bathroom means that you get a wicket.
There are 11 batsmen on a side in game of cricket, so when you have been to the toilet 11 times than you can't score any more runs and are out of the game.
To win the game you must have the most number of runs with the least amount of wickets.
You can 'declare' at a certain score, but are then not allowed to drink for the re-mainder of the session. Amoungst my circle of friends I am the current record holder scoring 66 runs for 0 wickets, (6 runs was equal to 425ml of beer @4.9% alcohol)