Use update panel like below :
<asp:UpdatePanel ID="Up" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:CheckBoxList
ID="chkdecisionlist" runat="server"
RepeatLayout="OrderedList" AutoPostBack="True"
OnSelectedIndexChanged="chkdecisionlist_SelectedIndexChanged">
<asp:ListItem>Preview Weekend or Preview Day </asp:ListItem>
<asp:ListItem>Newspaper/Magazine Ad </asp:ListItem>
<asp:ListItem>Social Networking </asp:ListItem>
<asp:ListItem>MBA/Higher Education Fair </asp:ListItem>
<asp:ListItem>Rankings</asp:ListItem>
<asp:ListItem>Other Media </asp:ListItem>
</asp:CheckBoxList>
</ContentTemplate>
</asp:UpdatePanel>
If still its not solve then use trigger in update panel.
Accept and vote if help to you.