Although an old question, I think it deserves the mention of one more solution.
If you do not wish to rely on hidden buttons or the illusive __doPostBack, there is the option of "ClientScript.GetPostBackEventReference", as described on this (likewise rather old but still great) page:
https://web.archive.org/web/20211020103534/https://www.4guysfromrolla.com/articles/033110-1.aspx
In short, if your UpdatePanel is declared like this:
<asp:UpdatePanel ID="MyUpdatePanel" runat="server">...</UpdatePanel>
then in JavaScript you can call the script that is generated by this server side code:
ClientScript.GetPostBackEventReference(MyUpdatePanel, "")
So in your aspx page you could have something like this:
function MyJavaScriptFunction(){
doSomeStuff();
<%=ClientScript.GetPostBackEventReference(MyUpdatePanel, "")%>
}
The code between <% and %> will be replaced with an actual javascript call when the page is parsed, so you can have a look at it by viewing the source code of the page in your browser.
It may not be easier than the other replies, but I prefer it since it does not introduce any extra elements and __doPostBack feels like a hack. :-)