You should always use a stripper before applying a new coat of stain to your deck.  A stripper will remove old sealers and stains.  Decks.com recommends using a deck stripper product from American Deck Products .  Simply dissolve the granular formula into water and apply across your deck with a mop, handled brush, or sprayer.  Wait 10 minutes and wash away the stripper with a pressure washer or a garden hose.