Here is a code snippet I copied from a question I answered at ASP.NET forum:

If the source columns are not matching with destination in position and/or name, you need to define ColumnMappings during the bulkcopy.

protected void Page_Load(object sender, EventArgs e)
   {
       // Connection String to Excel Workbook
       string excelConnectionString = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\property.xls;Extended Properties=""Excel 8.0;HDR=YES;""";

       // Create Connection to Excel Workbook
       using (System.Data.OleDb.OleDbConnection connection = new System.Data.OleDb.OleDbConnection(excelConnectionString))
       {
           System.Data.OleDb.OleDbCommand command = new System.Data.OleDb.OleDbCommand
           ("Select CountyCode,PropertyAppraiserName FROM [county$]", connection);

           connection.Open();

           // Create DbDataReader to Data Worksheet
           using (System.Data.OleDb.OleDbDataReader dr = command.ExecuteReader())
           {
               // SQL Server Connection String
               string sqlConnectionString = "Data Source=.; Database=test_for_forum;Integrated Security=True";

               // Bulk Copy to SQL Server
               using (System.Data.SqlClient.SqlBulkCopy bulkCopy = new System.Data.SqlClient.SqlBulkCopy(sqlConnectionString))
               {
                   bulkCopy.DestinationTableName = "tblPropertyAppraiser";

                   //Define ColumnMappings: source –destination

                   bulkCopy.ColumnMappings.Add("CountyCode", "DesCountyCode");
                   bulkCopy.ColumnMappings.Add("DesPropertyAppraiserName", "PropertyAppraiserName");
                  

bulkCopy.WriteToServer(dr);
               }
           }
       }
   }

http://forums.asp.net/t/1405240.aspx