This relates to lots of high-tech items, not just the obvious.
The obvious is of course hard disk drives (HDD) on your PC and laptop. This is easy, download one of the many freeware 'eraser' programs available on the net, just use your favourite search engine to search for 'free data eraser'. Most of these utilities will overwrite your data either 1, 3, 7, or the highest Gutmann level at 35 passes, not only do they erase the data, but they write a load of jumbled nonsense in it's place, making it economically impossible to read any of your data. But as the price of HDD's is so low these days, it would be sensible to remove the HDD and put a hammer to it.
But don't forget the not so obvious, sell a digital camera on eBay, and it could be very embarrassing if the buyer undeletes the pics of your other half (or you) in a compromising pose. Ideally, connect the camera or memory card to your PC and use the eraser program to overwrite all your old pics (refer to your handbook or online support as some rare and obscure cameras write system files to the removable memory card, so wiping the card may corrupt the camera), if that's not possible, then delete all the pics on the camera, put the camera on the highest resolution and take pictures of your carpet, then delete, then repeat again. Sounds mad, but it works.
Don't forget to delete the memory from MP3 players, games console units, digital tv boxes and tv recorders with a hard drive, as selling these on with information on may breach intellectual copyrights. Just use the delete/erase function on the unit to stay legal.
It is best practice to sell a mobile 'phone without the simcard, even if you do sell it without a sim, or with a new one, you still need to make sure the 'phone hasn't stored any of your numbers or messages in the 'phone memory, also, erase your photos' and videos. It's a good idea to reset the 'phone to 'factory settings' before you sell it, this will wipe all your downloaded ringtones, wallpapers. This applies to modern home 'phones too, as they store your numbers in their memory.
Don't forget to delete your favourites, home and recently visited places from your sat-nav prior to sale too.