Don't Trade a Dollar for a Penny
I keep losing games because I make small incursions -- while I'm ahead -- into enemy territory, leaving large holes open in my structures. Then, once I've lost sente, my opponent will strike at these holes, and I will lose big fields of moku. I've gained some pennies, but lost many dollars.
This is also known as "Penny wise, pound foolish" among people familiar with British currency.