Properly Releasing Excel From Memory

Question

I am using C# & VS2013 and VSTO. My code executes to perfection, but the one issue I have is that when I go to Task Manager and look at Details, Excel is still showing as running even after, I feel like I have properly cleaned up.

This is the syntax I use to cleanup and close all references to Excel. Just the references to Excel & VSTO, I left out the meat and gravy that does the leg work or this post would be to long.

Do I have something set-up improperly? Why does Excel still show up in Task Manager?

namespace ReleaseExcel
{
  public partial class Form12 : Form12
  {
    public static Excel.Worksheet newSeet;
    public static Excel._Worksheet currentsheet;
    public static Excel._Workbook oWB;
    public static Excel.Application xlApp;
    public static Excel.Workbook xlWorkBook;
    public static Excel.Worksheet xlWorkSheet;
    public static Excel.Workbook activeworkbook;

    public Form1()
    {
      InitializeComponents();
    }
    private void DoThis()
    {
        //Showing Declerations to Excel
        xlApp = new Excel.Application();
        xlWorkBook = xlApp.Workbooks.Add(misValue);
        xlWorkSheet = (Excel.Worksheet)xlWorkBook.Worksheets.get_Item(1);

        //Do some more stuff here

        //Showing a few more Declerations to Excel
        currentsheet = (Excel._Worksheet)(xlWorkBook.ActiveSheet);
        Excel.Range last = currentsheet.Cells.SpecialCells(Excel.XlCellType.xlCellTypeLastCell, Type.Missing);

        //Do a bit more stuff
        ClearExcel();
        while (System.Runtime.InteropServices.Marshal.ReleaseComObject(xlWorkSheet) != 0) { }
        while (System.Runtime.InteropServices.Marshal.ReleaseComObject(currentsheet) != 0) { }
        while (System.Runtime.InteropServices.Marshal.ReleaseComObject(xlWorkBook) != 0) { }
        while (System.Runtime.InteropServices.Marshal.ReleaseComObject(xlApp) != 0) { }
    }
    private static void ClearExcel()
    {
        GC.Collect();
        GC.WaitForPendingFinalizers();
    }      
  }
}

EDIT
I added in GC.Collect() & GC.WaitForPendingFinalizers() but now my syntax hangs on

while (System.Runtime.InteropServices.Marshal.ReleaseComObject(xlWorkBook) != 0) { }

It has been sitting on this line for about 5 minutes now, what causes the freeze?

EDIT # 2
Still no such luck. This is code

namespace ReleaseExcel
{
   public partial class Form12 : Form12
  {
    public static Excel.Worksheet newSeet;
    public static Excel._Worksheet currentsheet;
    public static Excel._Workbook oWB;
    public static Excel.Application xlApp;
    public static Excel.Workbook xlWorkBook;
    public static Excel.Worksheet xlWorkSheet;
    public static Excel.Workbook activeworkbook;

    public Form1()
    {
      InitializeComponents();
    }

    public void btnPush_Click(object sender, EventArgs e)
    {
        DoThis();
        GC.Collect();
        GC.WaitForPendingFinalizers();
    }
    private void DoThis()
    {
      //Showing Declerations to Excel
      xlApp = new Excel.Application();
      xlWorkBook = xlApp.Workbooks.Add(misValue);
      xlWorkSheet = (Excel.Worksheet)xlWorkBook.Worksheets.get_Item(1);

      //Do some more stuff here

      //Showing a few more Declerations to Excel
      currentsheet = (Excel._Worksheet)(xlWorkBook.ActiveSheet);
      Excel.Range last = currentsheet.Cells.SpecialCells(Excel.XlCellType.xlCellTypeLastCell, Type.Missing);

    }
  }
}