If you post, I will:
a) Tell you why I friended you.
b) Associate you with something -- a song, color, photo, etc.
c) Tell you something I like about you.
d) Tell you a memory I have of you. (This could be fuzzy, ye are warned.)
e) Ask you something I've wanted to know about you.
f) Tell you my favorite userpic from your list.
g) In return, post this on your own Livejournal. (Or not. Totally up to you.)