The 747 has a service ceiling of flight level 430 or 43,000 feet. This is the highest altitude that the aircraft can attain and still keep a climb rate of 100 feet per minute. Most other airliners have a similar ceiling ranging from 40,000-45,000 feet. However, commercial aircraft do not typically cruise at their service ceiling as this is not always economical. In my experience flying the 747, the average cruising altitude is usually about 35,000 feet. When loaded with full fuel, passengers (or cargo in my case) it is very difficult and wastes a lot of fuel to climb to 40,000+ feet. Oftentimes, large airliners like the 747 will take off and climb to 35,000 and cruise for a few hours then climb to 38,000 or 39,000 feet. The route also determines the cruise altitude that can be economically attained. A smaller airliner like the MD-82 or DC-9 may be capable of the same high altitude but due to the short route may not climb to it. It's easier to climb to 25,000 feet and cruise for an hour versus spend a long time climbing to 40,000 feet only to start a descent 20 minutes later. The key of cruising speed is to find the altitude which burns the least amount of fuel in relationship to the time it takes to travel the distance. Its a bit of number crunching that occurs long before the pilots even sit down in the cockpit to buckle their seatbelts and program the computers. Remember that the higher you climb, the thinner the air becomes and the more power you need to maintain lift. If you go too high, you'll burn too much fuel just to keep the plane at cruise speed. Also note that at these higher altitudes at max cruising speed you risk running into problems with the sound barrier shockwave.