Android-iOS-ASP.NET-WCF Authentication sharing

i'm developing an application that will have a Web, Android and iOS (maybe even Windows Phone) front-end, so we decided to place the business processes on a WCF Service Application, so we only need to write it once.

The problem is authentication, what's the best way to implement it?, i've tried some examples that i found googling, but i couldn't make any of those work.

Thanks in advance.