Bitmap myBitmap = null;

void OnPaint(...)
{

if (myBitmap == null)
{
    // create bitmap and draw to it
}

// copy bitmap to e.graphics

}


void SomethingThatInvalidates()
{

   // if for some reason your bitmap needs to change...

   myBitmap.Dispose();
   myBitmap = null;
}