Tip 1 - Pick your time.
If you can possibly help it, don't sell your car after Christmas, when no one has any money.

Tip 2 - First Impressions are everything
Just like you are going on a date - first impressions are everything. Get all the rubbish and junk out of it. Deodorize it. Wash it, clean it.

Tip 3 - As the good boy scout says. Be prepared.
Get your paperwork in order. Find the spare keys etc.