<input type="submit" name="myButton" value="Do something"
    onclick="javascript:WebForm_DoPostBackWithOptions(
        new WebForm_PostBackOptions('myButton', '', true, '', '', false, false))"
    id="myButton" />

$('#myButton').click(function () {
    // do some client-side stuff

    if (someCondition) {
        // don't postback!
        return false;
    }
});

<script type="text/javascript" src="Scripts/jquery-1.7.2.min.js"></script>
<script type="text/javascript">
    $(function () {
        var ploopButton = null;
        $("#ploop").click(function (e) {
            ploopButton = this;
        });
        $("#form1").submit(function (e) {
            if (ploopButton !== null) {
                ploopButton = null;
                return confirm('are u sure?');
            }
            return true;
        });
    });
</script>
<form id="form1" runat="server">
    <asp:Button ID="ploop" Text="ploop" runat="server" OnClick="ploop_Click" />
</form>

$('#myButton').click(function (event) {
    //Call prevent default on the event to prevent submitting the form
    event.preventDefault();
    // do some client-side stuff

    if (someCondition) {
        // don't postback!
        return false;
    }
});