Forms Data Controls :: Dropdown List Bind With Database Inside Gridview

I have created an asp.net customizable gridview where user will be able to add and delete data. Now I face some proplem with this gridview. now I want to add a dropdown list (which is dynamically bind with database) inside the grid. I created code and it bind well. when the page is post back previous selected dropdown list value lose and set to selectedIndex=0; For this reason I used if(!page.Ispostback) and put the dropdown binding code inside this. Here I faced a problem. The problem is when I want to add data inside grid the dropdownlist missin bind. How can I overcome this problem? For your kind observation I given entire code below:

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False"
CellPadding="4" ForeColor="#333333" GridLines="None"
onrowdeleting="GridView1_RowDeleting"
onrowdatabound="GridView1_RowDataBound">
<RowStyle BackColor="#EFF3FB" />
<Columns>
<asp:TemplateField HeaderText="SL">
<ItemTemplate>
<asp:Label ID="Label1" runat="server" Text='<%# Bind("RowNumber") %>'></asp:Label>
</ItemTemplate>
<EditItemTemplate>
<asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("RowNumber") %>'></asp:TextBox>
</EditItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Material">
<ItemTemplate>
<asp:DropDownList ID="drpMaterial" runat="server">
</asp:DropDownList>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Quantity">
<ItemTemplate>
<asp:TextBox ID="txtQuantity" runat="server"></asp:TextBox>
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"
ControlToValidate="txtQuantity" ErrorMessage="Required Field Missing">*</asp:RequiredFieldValidator>
</ItemTemplate>
</asp:TemplateField>...............................