Have you ever been presented with a dish so intoxicatingly delicious that you felt you couldn't stop eating it? That you only did stop when it was gone? Think back: Were the last few bites as enchanting as the first few? Thomas Keller would guess not. When we spoke for the Smart Chefs book, the celebrated chef-owner of Per Se and the French Laundry shared an interesting perspective on how much is just enough. 
photo by Deborah Jones, TKRG
"Our whole menu is based on the law of diminishing returns. The most compelling portion of a dish is in the first three or four bites. With the first bite you're getting into it, by the second bite you start to realize it, and it is at the third or fourth bites you get the maximum appreciation and pleasure from that dish...and you keep eating because of that memory of it being really extraordinary. But was it as good [at the end] as it was at that second, third or fourth bite? No."
His solution? Smaller portions. Don't allow yourself to keep eating and eating trying to recapture that early blush of pleasure. The time to stop is when you still are so excited by a dish that you want a little more. Keller has long served a signature salmon cornet canapé, and people often ask for a second one after they have demolished the first delicate cone studded with black sesame seeds and filled with sweet red onion crème fraîche and salmon tartare. Sorry, folks, no more cornets for you. "No," says Keller. "It's a matter of finishing a dish at the height of flavor impact." Of course, at that moment he's sending out another small enchantment. 
salmon cornets. photo by Deborah Jones, TKRG
I try to remember this when I'm getting over enthusiastic about a restaurant meal. But it also works at home, though there you need to be even more disciplined. You need to be both the consumer in want of that second helping, and you have to be your own Chef Keller, and politely tell yourself: No. Sometimes I have to remind myself that, he's right, I'm just chasing the memory of that initial yum. Better to have less, savor more.