Crystal Reports :: Parameter Passing From Previous Page?

I have 2 asp.net pages first page (PaySlipForm.aspx) contains ui elements for getting values from user, second page shows the report depending on the parameters passed from first page. The problem is first time the report shows correctly but when I go back on first page by clicking the back button of browser and changes the selection the report is shown empty only labels are shown.

First Page PaySlipForm.aspx
<asp:DropDownList ID="ddlAllEmployees" runat="server" style="margin-left: 7px"
Height="20px" Width="174px"> </asp:DropDownList>
<br />
<asp:DropDownList ID="ddlMonth" runat="server" Height="23px">
</asp:DropDownList>
<br />
<asp:DropDownList ID="ddlYear" runat="server" style="margin-left: 18px"
Height="23px" Width="77px">
<asp:ListItem>2010</asp:ListItem>
<asp:ListItem>2011</asp:ListItem>
<asp:ListItem>2012</asp:ListItem>
</asp:DropDownList>
<br />
<asp:Button ID="btnShow" runat="server" Text="Show" PostBackUrl="~/Reports/PaySlipViewForm.aspx"/>
Second page is PaySlipViewForm.aspx that has the code
<CR:CrystalReportViewer ID="CrystalReportViewer1" runat="server"
AutoDataBind="True" EnableDatabaseLogonPrompt="False" Height="50px"
ReportSourceID="CrystalReportSource1" Width="350px" />
<CR:CrystalReportSource ID="CrystalReportSource1" runat="server">
<Report FileName="../Reports/PaySlip.rpt">
</Report>
</CR:CrystalReportSource>
---------------------
if (!IsPostBack)
{
int employeeID = Convert.ToInt32((((DropDownList)Page.PreviousPage.FindControl("ddlAllEmployees")).SelectedValue));
int monthID = Convert.ToInt32((((DropDownList)Page.PreviousPage.FindControl("ddlMonth")).SelectedValue));
int year = Convert.ToInt32((((DropDownList)Page.PreviousPage.FindControl("ddlYear")).SelectedValue));
CrystalReportViewer1.SelectionFormula = "{Payroll.EmployeeID} = " + employeeID + " And {Payroll.Month} = " + monthID + " And {Payroll.Year} = " + year;
}