<asp:DropDownList ID="yourID" runat="server" AppendDataBoundItems="True">
<asp:ListItem Value="" Text="" />
The key here is the AppendDataBoundItems="True" property. This allows you to define your own list items, and then any items that are bound to data are added after those items, aka appended.
However, if you find that you're getting the same items added multiple times you can do it this way instead. Leave AppendDataBoundItems="False" and rather add the empty/blank entry programmatically. To do that add the following code after your drop-down has been filled
DropDownList1.Items.Insert(0, new ListItem(String.Empty, String.Empty));
DropDownList1.SelectedIndex = 0;
This will add an empty/blank entry at index zero of your dropdown box, and also make sure that it is the selected value. If you don't want to change it to the selected value don't use the second line. I found this on http://stackoverflow.com as well