Filling <device> (eg: /dev/sda3) with random data before encrypting it greatly increases the strength of the encryption. The downside is that it can take a very long time.
Warning
The commands below will destroy any existing data on the device.
The best way, which provides high quality random data but takes a long time (several minutes per gigabyte on most systems):
dd if=/dev/urandom of=<device>
Fastest way, which provides lower quality random data:
