It might be a bit controversial, and in some parts of the country might even get you fined for contempt, but if you're male you could do it the way the Romans did it, before anybody used Bibles:

With your left hand, hold your testicles and swear on them.

Seriously, that's why it's called testimony.

I am told (not overly politely) that according to's Latin etymology section, this is a legend. Duly noted, but pending further investigation I will take the word of my Latin teachers.