Question

your best friend always litters around the desk what can you do to motivate her/him to stop littering

Answer 1

go and talk just listen and at the end speak to him 
 why do you want do such a things in front of every one ?
won't you realize that if you will do like every one will see as dog ?that is litering around? 
ask him related to that  and make him feel shame and create a hate rate in hi, to stop continuous in work not any more.
every one can do this

Answer 2

first we just tell him two or three times if he doesnt listen try to make him understand what are the disadvantages of doing like this.
even now if he doesnt listen to you just try to say"you know people always comment on those who litter and u know what they comment they say see that animal how he is littering "
U MAY FEEL BAD WHILE SAYING THIS TO YOUR FRIEND BUT TO MAKE HIM NOT TO LITTER YOU NEED TO SAY THIS