I tie my clousers and other baitfish patterns with guards and rarely loose them to the kelp. I have successfully added them to flies after the fact too. With a little practice it only takes a couple of minutes.
Wire weed/snag guards can be added to many flies after the fact. These additions wont be as secure as a guard added during fly construction but will still be functional. Guards made from single strand trolling wire, #3, #5, or #7 depending on hook size, can be added to most baitfish patterns. When in doubt, use #3 wire. Look up the Eagle Claw 449w hook for an example of what these will look like when fitted.
Cut a length of wire and fold in half. Make it look like the wire you see in a photo of the 449w. You need a little extra wire to extend past the eye of the hook. The extra length can be secured on the opposite side of the fly head from the hook with the wire run through the eye and then bent back so that it catches on the hook point. The bend that results provides a spring that holds the the wire were you folded it against the hook point. Experiment with the degree of bend/curve/spring in the wire. You don’t want so much tension that the fly becomes fishless, but you want enough that it wards off most kelp and rock. You can attach the guard on the same side as the hook, without running it through the eye, but it will not be as secure. This method can be used for flies tied on jig hooks.
If the fly has a bulky head or a head coated with epoxy or similar materials the addition my be more difficult or not possible. With epoxy or other similar finishes it may be necessary to rough up the surface of that nice head with sandpaper or a file so that the thread will grip and not slide to the hook eye when wrapping.
Maybe I can find some time to post a photo sequence this week, but no promises.
In the fly tying instructions on this web site for Tomaso’s Jetty Fly, a method of adding a single strand wire guide is shown. These method can also be used to add a guard after the fact and is pretty easy. The disadvantage of this method is that the lead will sometimes get wrapped on the the guard and the fly will not ride correctly until the problem is corrected.