K.L.
Ok, I know this is kind of extreme to some people...what about getting matching best friend tattoos? I have no idea how you or your best friend feels about tattoos, so I'm sorry for suggesting this if you hate them!!! But, they can be very tasteful and easily hidden...as the 3 I have are. It would definitly bond you two forever!
Or if you aren't into tattoos...:)...I love Pandora jewelry! The bracelets are beautiful and you can add different charms. As friends, the two of you could add a new charm each year...buy each other one for birthdays. There are so many to choose from!