In my veggie garden, I've had good low-tech luck with a bunch of small sticks about 12" long stuck into the ground about 8" apart. I used my fruit-tree prunings that I had put in the kindling bucket, along with some longer bamboo stakes mixed in. It discouraged the cats long enough for the plants to get established.
I've also laid chickenwire in perennial beds, planted thru it and covered it with a nice layer of mulch. The first time they step on it, they learn they don't want to be there after all.
