Robin Harris, writing for ZDNet:

With Mac OS 10.3 – El Capitan – Apple has removed the Secure Empty Trash function. Why? Because with solid state drives – SSDs – Apple couldn’t guarantee that a secure erase would actually erase the data.

Thus when you sell a Mac – almost all of which have had SSDs for the last 4 years – using Secure Empty Trash with an older version of OS X, or trusting Empty Trash to nuke your data is asking for trouble. Follow these instructions and your data will be well and truly vaporized.

The article walks you through the process. Tuck this one away, pass it along.