As the others have said, writing zeroes should be sufficient. You can also write random data, or do a secure wipe, if you're really paranoid.
Someone wrote an open challenge to anyone who could recover data from a drive that was overwritten with zeroes, and also contacted 3 data recovery firms for the challenge:
The article includes a paraphrased conversation with someone at one of the data recovery firms contacted for the challenge:
"According to our Unix team, there is less than a zero percent chance of data recovery after that dd command. The drive itself has been overwritten in a very fundamental manner. However, if for legal reasons you need to demonstrate that an effort is being made to recover some or all of the data, go ahead and send it in and we'll certainly make an effort, but again, from what you've told us, our engineers are certain that we cannot recover data from the drive. We'll email you a quote."