Wooden swing sets and other play structures frequently become wobbly through regular play, long before they have lived out their usefulness. Random bracing and repairs can do more damage than good. Adding appropriate braces can add years to your swing set's usefulness. Use the same kind of wood the frame is made from, if possible, since it will weather to the same color as the original swing set.
Tighten all loose screws and bolts with a cordless drill or socket wrench. Remove hardware that spins in the wood, since the threads are stripped. Take the original hardware with you for comparison and buy bolts or screws slightly thicker and longer than the originals. Replace the old hardware with them. This will go a long way toward bracing the swing set and realigning the parts correctly.
Use steel mending plates and L-brackets to reinforce weak joints. Place them underneath cross-members, like the top bar and horizontal bars in the A-frame ends of the swing set. Use galvanized hardware and heavy-duty treated deck screws to attach the brackets to ensure the braces will last. Use the longest screws you can to make the joints as strong as possible.
Cut 45-degree miters on the ends of 2-by-4 lumber pieces long enough to brace the corners of the top bar. Predrill the lumber braces with a 1/8-inch drill bit. Use 3-inch treated deck screws to attach the braces to the side frames of the swing set and the other side of the top bar to brace the frame solidly. Countersink the screws so that the heads are just below flush with the surface of the braces. Use at least two screws at each end of each brace.
Cut treated or cedar lumber to fit between the legs of the swing set at the front and back. Fit the braces between the legs and bore through the legs from the outside into the ends of the lumber braces. Drive long lag bolts through the outside of the legs into the ends of the lumber braces to tighten them. Fill the area between the braces with sand or round pea gravel to add safety to the swing set in case of falls and to add stability to the bracing.