Wooden gates are obviously an important part of your wooden fence, but because they are the most used part of the fence, and hopefully the only section that moves, they are usually the first part of the structure to suffer wear and tear. If you notice your wooden gates sagging or otherwise not fitting properly with the rest of the fence, consider the following points to be able to troubleshoot and fix the problem.
If you notice your wooden gates are sinking or crooked, first inspect the screws holding the hinges in place. Chances are some of the screws are either missing or have worked themselves out partly. Tighten up the screws and then monitor them over the next few weeks. If you notice the screws falling out again, choose different screws, maybe larger ones, or add a nut to keep them in place.
Your wooden gates could be sagging because the hinges themselves are broken or worn out from too much use. If changing the screws does not seem to straighten out the gates, it may be time to replace the hinges themselves. Go for more robust hinges that will last longer than the previous ones and use heavy-duty screws or bolts to keep them in place.
The wooden gates will be attached to posts, which generally should be bigger and sturdier than the rest of the fence posts. Sometimes, the gates will sag, sink, or become crooked because the posts are no longer straight. To fix the gate posts, first remove the gates and then either remove the posts to re-set them completely, or simply push them into a vertical position. To reinforce the posts, you can pile dirt or rocks around them or even pour some cement to make sure they stay where you put them. Re-attach your gates, and they should no longer be sagging.
If your wooden gates have no kind of extra support, they could sag no matter how straight the posts are or how tough the hinges are that hold them to the rest of the fence. If your wooden gates are simple solid pieces of wood or only have thin decorative wooden beams, you'll need to add some kind of bracing or support. Do this by attaching sturdy pieces of wood to the back of your gates, either a single piece diagonally, three pieces in a "Z" shape or two pieces in an "X" shape. If you elect to add a single diagonal support, make sure you attach it from the bottom hinge at the gate post up to the opposite side of the gate.
Another fairly straightforward way to secure your sagging wooden gates is to attach a tension rod to each gate. This works similarly to a diagonal wooden brace, but is made of cable and uses tension to hold the gates straight. There is a turnbuckle in the middle of the cable which you can tighten to increase the tension if needed. These turnbuckle tension rods are available in most home improvement stores.