1. Wipe your hard drive with a professional tool that is certified for hard drive erasure. Tools such as Blancco are 100% certified to make your data irretrievable, whilst being able to reuse the same hard drive (hence being eco-friendly).
2. Physically take the hard drive out and smash it to smithereens. The main part of the hard drive, once you unscrew it, is called the platter. You can hammer this, burn it or destroy it anyhow you want.