How can I reliably delete my data from a drive?

Just deleting your files and folders through the OS does not really remove your data from a drive.

All it does is to remove the entries for your files and folders from the table of contents and reclaim the space used by your data as "free space", ready to be used again.

Only physically overwriting your data with other data or zeroing it out will reliably delete your data.

Contrary to popular belief, it is not necessary to overwrite your data more than once, as a research paper has recently shown:

http://blogs.sans.org/computer-forensics/2009/01/15/overwriting-hard-drive-data/