Software/utility to completely erase cookies/cache
The easiest way is to use a free (or paid) utility to do it for you.

Onyx is a free one that does the job, you have an option to clear your browser's different values: 



Cocktail is a paid ($14.95) nice utility too, that does what Onyx does and the UI is a little bit more elegant and give you some more flexibility:



Of course, you will have to make sure that you securely empty your thrash if you really want to make sure nobody has access to deleted stuff. 

Cocktail has a nice option that may interest you in this regard:



…which should cover your secure concerns :) 

(if OnyX has a secure delete, I couldn't find it in the preferences).