I personally don't think using BMI is the best way to go if you're happy with your weight. It's a useful tool as a statistic, but becomes less useful for describing/prescribing individuals. If all your health indicators are good and your doc is happy with how you're doing, I think it's a great idea to focus on more fitness-based measures. Here are some ideas:
-Measure yourself. I heard about myshape.com on here, where you can enter about 2 dozen measurements and it helps you find clothes that will fit you. You can keep track of your muscle circumferences and get cute clothes at the same time
-Take up running and see how your pace improves. The Fat or Fit guy swears by using pace as an indicator of fitness. As a slow runner, I don't totally agree, but seeing improvements in your pace/endurance can be very telling and rewarding
-I started keeping track of my bf%. I started at 33.3% 18 months ago and am now around 22-23%. I lost a percent just in the last month, even though my weight has only gone down about 3-4lbs.