I'm a personally a huge fan of Del.icio.us, and it's Firefox addon. I don't even use the regular Bookmarks option in my browser anymore and use that instead. That way I have access to all of my bookmarks, tagged, from anywhere I want.
Anyway, one thing about the addon that has always irked me is that I had to manually check the "Mark as Private" checkbox every single time I added a new bookmark, since I like to keep all of mine private. But no mas! Here's a slick tip on setting up the addon to automatically check the Mark as Private for you.
If you don't have the Del.icio.us addon already, grab it here: Clicky
Then just follow these easy steps:
All done! from now on whenever you add a new link with the Del.icio.us addon, it'll have the "Mark as Private" checkbox automatically checked (as seen in the example screenshot below). How sweet is that?