Compliment the poster above you, easy.
Example...
Poster #1 - I like your avatar
Poster #2 - I like your signature.
Poster #3 - I like that you've helped me out back there.
Avoid this...
Poster #1 - I like that you liked my avatar
Poster #2 - I like that you liked his avatar.
Poster #3 - I like that you like that he liked his avatar.