set the height and width of the list box and set over flow as scroll as below.
<asp:listbox id="lstBox" runat="server" height="50px" width="100px" style="overflow:scroll;" xmlns:asp="#unknown">
this will maintain the scroll position of the list box to the previous selected position.
this is optional, this got nothing to do with list box scroll bar position. this is for overall page scroll position. better you can set this, so that no need to scroll down again to that position after post back.
hope it helps.