The gate in the Houston house's back yard became unhinged during Hurricane Ike a few weeks before we moved in, and the previous owner did such a lousy repair job that I had to replace the replacement within a couple of years. The replacement gate fit fine and I did a good enough construction job that it doesn't sag, but the BigBox lumber I used to build it began to warp seemingly within minutes, causing a serious case of gaposis near the top. I decided that a set of eye bolts, some cable and a turnbuckle would straighten it out, so I returned to my local BigBox store where I'd bought the lumber (blue, not orange). There on the hardware aisle I found a National Zinc Anti-Sag Gate Kit (V-852) and brought it home.
The kit consists of a two zinc-plated steel support brackets that fit over opposite corners of the crosspieces plus screws to install them; several feet of 1/8-inch galvanized steel cable, plus a pair of galvanized steel rope clips (also called cable clamps); and a ¼" turnbuckle with hook-end bolts. The turnbuckle is aluminum and the bolts are zinc-plated steel.
Installation
1) Use the included screws to install one corner bracket at the hinge end of the top crosspiece and the other at the post end of the bottom horizontal. I did it the other way, since the gate didn't sag: it was warped outward at the top.
2) Open the turnbuckle as far as it will go and place a hook through the eye hole hole in the top bracket.
3) Make a smallish loop at one end of the cable and secure with a rope clamp. Place this loop on one of the hooks in the turnbuckle. If your turnbuckle has only one hook, thread the cable through the eye screw before you secure the loop.
4) Thread the other end of the cable through the eye-hole on the bottom bracket. Make a second loop and pull it as tight as possible, then secure it with another rope clamp.
5) Screw down the turnbuckle until the cable lifts the sag out of the gate or, as in my case, unwarps the warp.
turnbuckle to take out some fresh warp.
An anti-sag kit is an easy-to-install and economical solution for problem that's altogether too common. This or a similar kit is sold under several labels, such as the Stanley 76-0828 and the Ace Hardware 5299029. Some have an eye bolt instead of two hooks on the turnbuckle, but as long as the parts are zinc they're pretty much all the same.