how can I tell the browser to cache the result of a webmethod called with AJAX?

I am doing AJAX using the asp.net ajax library. I make calls to web methods (or web services??) and get the return value on the client:
UserService.GetUserProfileFlyoutHTML(this.GetUserProfileFlyoutHTMLCompleteSuccess, this.GetUserProfileFlyoutHTMLCompleteError, this);
GetUserProfileFlyoutHTMLCompleteSuccess: function(results, context, methodName) { },
GetUserProfileFlyoutHTMLCompleteError: Go to the complete details ...