How to prevent double click on button in ASP.Net

Hello all i have a asp code as

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
    <asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />
    <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ErrorMessage="please enter any character" ForeColor="#ff3300"
    ControlToValidate="TextBox1"></asp:RequiredFieldValidator>
    <br />
    <asp:Label ID="Label1" runat="server"></asp:Label>

and i want to stop the user for clicking same button multiple times during page postbacks so the code written as

static int clicked = 0;
    protected void Page_Load(object sender, EventArgs e)
    {
        clicked = 0;
        string strProcessScript = "this.value='Processing...';this.disabled=true;";
        Button1.Attributes.Add("onclick", strProcessScript + ClientScript.GetPostBackEventReference(Button1, "").ToString());
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        ++clicked;
        Label1.Text = clicked.ToString();
    }

it works fine..but the validation does not work.. and if we comment the lines in page load i.e.

string strProcessScript = "this.value='Processing...';this.disabled=true;";
        Button1.Attributes.Add("onclick", strProcessScript + ClientScript.GetPostBackEventReference(Button1, "").ToString());

then the validation works, but then user can click button multiple times.. how i can achive both