JavaScript
function DisableButton(buttonElem) {
buttonElem.value = 'Please Wait...';
buttonElem.disabled = true;
}
ASPX
<asp:button id="btnSubmit" runat="server" Text="Submit" />
C# Code
protected void Page_Load(object sender, EventArgs e)
{
btnSubmit.Attributes.Add("onclick", "DisableButton(this);" +
Page.ClientScript.GetPostBackEventReference(this,
btnSubmit.ID.ToString()));
}