I add 3 blank lines (hit Enter 3 times) into the 'Have your say' box, before inserting a quote.
After highlighting and copying the text you want to quote, place the cursor on the first blank line, then click the Quote button to insert a yellow box so as to paste the copied text into the yellow box.
When you want to add your own comments below the yellow quote box, place the cursor onto one of the other blank lines you had already first added. If you added the quote box to the first blank line, the other 2 blank lines will remain below the yellow quote box.