Training out the window...

My cat has long been well-trained to not get on countertops. Sure, he's had the slip up now and then, but overall, he's always been good about it.
Until recently. Apparently the allure of counters came knocking and was too wonderful to ignore. He's getting on every countertop in the house! What's worse is that now that he's older, he has the good sense to NOT do it when I'm around. If I walk into a room or he hears me coming, he leaps off the counter and races away before he can get in trouble-- and since I can't punish him after the fact, I'm at an impasse.
I've been trying to keep food off the counters so he doesn't get "rewarded" for jumping on them, but that isn't enough. Any thoughts or tips?

The most clever one I've heard so far (but I haven't tried yet), was to line your counters with soda cans that have pennies inside (tape them closed). That way he'll see when he jumps on the counter that it's a scary experience because of all the noise it makes!

Probably not the best option if you are USING the countertops at the time, but you said he only does it when you're not around... so it just might work!

There are also air sprayers that you can buy that give out a puff of air when they sense motion.

What about using aluminum foiler around the edges of some counters? I heard some cats do not like the sound or the feel of that when they trample on it.

Cats are natural climbers and love to explore. The suggestions given are good and will help discourage going on countertops. But a cat is a cat.