A bouncy deck can be annoying at the very least, and unsettling at worst. Even if you know your deck is up to code and most likely will not collapse beneath you, learning how to reinforce your deck may be a good idea for your own peace of mind.  Bouncy decks are usually caused by too few joists (the support beams that hold up the deck), and extra support can be added in a number of ways.

One way to fix a bouncy deck is to add another beam under the center of the deck across the joist span.  This is the most difficult way to achieve more support, as it requires digging cement trenches and you will probably need more than one person.  Also, adding another beam may not be practical if your deck is high above the ground.

Another option is to add more supporting joists to your deck.  You can either add the joists on a one-to-one basis, or add a second joist every third or fourth board.

The easiest option for most decks is "blocking".  With this method, you add solid wood blocks in between the joists.  The wood blocks should be separated from one another by three to four feet, and they should also be staggered.  You should make sure the blocks fit tightly, and are nailed in with galvanized nails.  It is important to note that this method may not be the best long-term solution for areas with large humidity swings.

If you are planning to replace your deck planks, you could improve stability by choosing thicker boards.  Not all boards are equal, and two-inch boards are much sturdier than 1-inch or 1.25-inch boards.

Another useful tenet to remember is that screws are always sturdier than nails.  So, if you're looking for a way to tighten up your deck without adding or replacing any wood, you could consider reinforcing every area where nails are used with screws.  This is a time consuming project, but it will improve the stability of your deck while also increasing its longevity. 

Bouncy decks commonly occur when there is too little support. Adding extra beams, joists and blocking are all excellent ways to create a sturdier structure. Replacing deck planks with thicker boards and using screws instead of nails may not completely correct the problem, but they can help create a more solid structure.  With a little extra time and elbow grease it is possible to create a more stable deck, while also increasing its lifespan.