ASP.NET asynchronous post-back on asp:button click

I am using AJAX in asp:net and am trying to handle a situation where an asp:button triggers both a javascript method and a codebehind c# method. To do this I am using onclick and onClientClick attributes for the button. I need a postback to occur for the codebehind to be called however this postback causes the javascript to not work because variables have lost state. Can anyone describe how to handle this? Possible with ajax and async postbacks? It’s starting to drive me a bit crazy!