新手问一个关于GridView带查询条件分页的显示问题，大家帮帮忙

aspx页是个带条件的查询，结果分页显示，刚载入页面时全部数据分页显示各页跳转都没问题，选择了条件再查，第1页的数据没问题，但点第2页时显示的却是不按条件查时第2页显示的数据，再点第1页时第1页也变成不带条件查询时第一页的数据了，查询条件不起作用了。。
<asp:GridView ID="GridView1" AllowPaging="True" runat="server" AutoGenerateColumns="False" CellSpacing="0" CellPadding="0" Width="572px" OnPageIndexChanging="GridView1_PageIndexChanging" PageSize="10" OnRowDeleting="GridView1_RowDeleting" OnRowUpdating="GridView1_RowUpdating">

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
ListItem li1 = new ListItem("请选择", "请选择");
DropDownList1.Items.Add(li1);
DropDownList2.Items.Add(li1);
for (int i = 1; i < 5; i++)
{
ListItem li = new ListItem(i.ToString(), i.ToString());
DropDownList1.Items.Add(li);
}
DropDownList1.DataBind();
DropDownList2.Items.Add(new ListItem("使用", "1"));
DropDownList2.Items.Add(new ListItem("未使用", "0"));
DropDownList2.DataBind();
}
...//定义的str_sql
DataTable dt = DataCommSQLServer.GetSqlData(str_sql);
GridView1.DataSource = dt;
GridView1.DataBind();
}

protected void Button1_Click(object sender, ImageClickEventArgs e)
{
string state = DropDownList2.SelectedValue;
string jb = DropDownList1.SelectedValue;
string code = TextBox2.Text;
string name = TextBox3.Text;
if (state.Equals("请选择"))
{
state = "";
}
if (jb.Equals("请选择"))
{
jb = "";
}
...//定义的str_sql
DataTable dt = DataCommSQLServer.GetSqlData(str_sql);
GridView1.DataSource = dt;
GridView1.DataBind();
}
protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
{
GridView1.PageIndex = e.NewPageIndex;
GridView1.DataBind();
}

第一次发提问贴，大家帮帮忙