You can try the 1-2-3 Method, from "1-2-3 Magic - Easy-To-Learn Parenting Solutions That Work From Dr. Thomas Phelan", their website
http://www.parentmagic.com/.
I've heard good reviews from mums using the method, and you can see one example in a Brillkids forum mum blog:
http://raisingethan.wordpress.com/category/parenting/discipline/My son has different attitudes with different people too. Maybe he knows or feels which people he can influence most or in which moments. Or at least he tries whenever he wants because the results are not always up to his expectations
![LOL](http://forum.brillkids.com/Smileys/emoji/lol.png)
Andrea