The other way, of course, is educating people to spay and neuter their own cats, in a timely manner, and to keep them indoors until the procedure is done.
TNR people also rescue mother cats and their kittens, and whenever possible, foster the kittens until they are old enough to be rehomed. The mother cats are spayed, and if not totally wild, are also rehomed wherever possible, otherwise, they are returned to the colony. There are times when pregnant cats are spayed, unless they are close to full-term.
There are a number of active TNR people in the About Cats forum, and I know a number of others personally and through email. Every year I've seen them approach the burnout point because of their despair over the sheer numbers of new kittens each season. There simply isn't enough time and resources to save them all. TNR/rescue groups are almost always short of funds, and often use their own hard-earned funds to pay for cat food, traps, veterinary care, and other expenses for their feral cat colonies.
Even though kittens are often adopted first, many of this year's kittens become next year's adult cats, still waiting for homes. Each year, at kill shelters, thousands of older cats, pregnant cats, and mother cats with newborn litters are euthanized to make room for the influx of new, more adoptable kittens.
And so it goes, every year... I have nothing but the utmost respect for these selfless people who really make a difference in the world of cats, often at the sacrifice of their own needs.
I Hope this helps explain TNR. You can find out a lot more about it in my Rescue Resources section.