Hey guys! Ingame i've seen alot of players that didnt know u can get the Alistar & Unchained skin, and Tristana & Riot Girl Tristana skin for free by just like Riot on Facebook, and to subscribe to Riot's Youtube channel!
Links are in the descriptions of these video's;
For Tristana:
http://www.youtube.com/watch?v=BVoL7k2KtIM
For Alistar:
http://www.youtube.com/watch?v=fVfclrhXdpA
Have fun!