Your first principle is dead on---that your brakes are absorbing your drive energy, and wasting it as heat. every time the pads touch. You have used fuel to gain kinetic energy, and by braking, you convert that energy to heat in your brake pads. instead of forward motion.
However, holding your clutch in is going to burn out your throw-out bearing. Just flick it into N and let the clutch back out. I'm assuming your car is a 4- or 5-speed. You can skip a gear or two on the way up, as long as you make a fairly gentle acceleration demand when in too high a gear. For example, when starting from a dead stop in low, just let the clutch out enough to get it rolling, and shift up to second, which will handle it as long as you have built up a bit of roll and don't try to accelerate too briskly.
I assume you do a lot of steady highway driving, as well. If the countryside is at all hilly, you can save gas using the "constant throttle" strategy---try to hold the foot pedal in the same place, so your car goes up hills slower and down faster. I hate cruise control, for a number of reason. One is that it tries too hard to suck your car up a hill at speed, which wastes gas. The second reason, unlrelalted to economy, is that it inclimes me to play chicken too much---if I will need to change lanes but have to wait for an opening, how close to the car in front can I go before I have to kick out of cruise? The third thing I dont like is the idea of turning over driver control to a machine, which reduces my sense of the road.
And, here's another fun road pastime: Watch for the highst license number in your state, and report it to
LicensePlates.cc: The license plate collector’s portal.