If you're working on extending your deck, there are a few things you can do to support it while you work. First, make sure the area where you're working is level. You can use a laser level or string line to make sure the area is level before you start working. Second, use 2x4s or 4x4s to support the deck while you work. Place them at regular intervals around the perimeter of the deck and in the middle of any spans longer than 6 feet. Finally, when attaching the new decking boards, use screws rather than nails so that they will be better able to hold up over time.