Basically, all the options people are suggesting works. The reason behind it is, to define how the inactivity wipe system works: A character with no action logged for 6 months will be automatically deleted from the database. Now with that in mind, people tend to pick whichever "action" convenient to them. Undocking, selling or buying cargo, entering a command, etc. are all considered logged actions and you simply need to do one of them to make sure your char will not be deleted for the next 6 months.

What I personally do is to log the char, sell and rebuy a nanobot, and log off. That's enough to keep the char "active", and there are a lot of accs I'm keeping alive for other people who're on a break from Disco. I don't fly them, and simply do what I mentioned above once every few months. None of the chars have been wiped even though some of those accs have not been used at all for over 2 years now.