Pitch trim creates a force balance that drives the aircraft to seek a specific angle of attack. But as weight is reduced, less angle of attack (AOA) is required for level flight.
So, as fuel is burned, with the airplane trimmed to maintain, say, a 4 degree angle of attack, static stability forces will tend to maintain that angle of attack. The reduction in weight means the lift generated at that AOA is excessive, and the plane will climb as it maintains its trimmed AOA.
To compensate, you have to trim to a lower AOA to regain the original lift/gravity equilibrium.
Retrimming to a lower angle of attack reduces drag, and so you also have to reduce thrust/power to keep the original speed.
What happens in practical terms is, you fly along, and we'll say you're hand flying and you have it trimmed hands off. The pitch attitude shows 4 deg nose up. After a little while you'll notice the plane starting to creep into a climb. You'll pitch the nose down slightly, to a slightly lower pitch attitude, say, 3.8 deg. If you do nothing else, the speed will start to creep up, so at the same time you'll reduce thrust a tiny amount to keep the speed constant.
If you're on an autopilot without autothrottle, you'll be observing and noticing that the airplane very gradually noses down slightly over time as the autopilot retrims to compensate for the weight reduction, and while that's happening you'll be easing the thrust back in tiny adjustments to keep the speed bug at the target IAS or Mach#. Or if there's an autothrottle system, you just sit and watch twiddling your thumbs.
From start to finish, depending on how much all that fuel weighed, you might find that the pitch attitude is maybe a degree lower at the end of the cruise than it was as the start, and thrust is a couple of percent lower as well, to maintain the same constant altitude and speed.
Flaps are not used because they are only for low speed use to reduce minimum flying speeds. Deploying even small angles of flap at cruise speed (airplanes generally aren't structurally designed to operate at cruise with flaps extended anyway) you end up with too much lift and a lot more drag, so efficiency goes to heck; there's no point.