Here's the process:
Check that the battery and alternator are doing their jobs. The battery should not be much more than 14V with engine running. If it's less, then the alternator needs attention. If it's more, and increases with revs, then the regulator (in the alternator) is stoofed.
Now, disconnect the +ve terminal from the battery. Connect an ammeter (multimeter set on Amps) between the +ve battery terminal and the +ve clamp. There should be a current flow indicated. Now start pulling fuses until the current suddenly drops. That gives you the circuit that is causing the problem. Now start disconnecting stuff on that circuit to find out which one is causing the problem.
A common cause of this problem is the light in the boot not turning off properly. Next would be the alarm, closely followed by the CD / radio (the battery backup of the settings). In combo with a faulty / old battery, any of these could drain the battery overnight.
Hope that helps.