My 99 Chevy Prizm uses a quart of oil about every 300 miles. No leaks or smoke. Any ideas?

There has to be either smoke or a leak. Have your Mechanic do a compression test on all cylinders. It will give you a lot of information on your engine

replace the valve stem seals

A "leak down" test will test the cylinder effectiveness, as will a compression test. These will tell you how good the engine is.

if it is not leaking then it is being burned by the engine, how many miles go by before you need to add oil?, if u are adding more than one quart every 1000 miles u have piston reing problems and or valve guide seal leaks. U won't see any smoke in most cases unless you are really going through the oil, like 1 quart every 300 miles.

Well, if there are no leaks, then it has to be piston rings or valve seals. If it's only burning a quart every 1500 miles, you would barely see any smoke, and by the standards of 20 or 30 years ago, that was normal.

If you love the car as you say than be prepaired to shell out the bucks for a rebuild or replacement engine, because thats the only fix for your situation. You may not see any blue smoke but believe me its burning it, or you got a leak witch you say you don't. Its going someware and most likely its out the exhaust.

cylinder head gasket???

Per previous post. A quart in 750 miles would be extremely high.  A Porsche 930 Turbo Carrera driven hard doesn't use this much unless it's motor is toast, and it is practically a race car.  You should expect at most a quart of oil consumption for 3000 miles.  A quart of oil in 750 miles is a quart EVERY OTHER tank of fuel, and would indicate a motor that needed a rebuild long ago (or seals, guides or rings).  BTW, If you can see smoke in the read view, it looks much worse when someone is driing behind you.  STP or changing to the next higher weight of oil are short term solutions that may help you not leave a trail of smoke.
If you need valve seals, maybe that's it,  but if you need guides, the rings are probably near shot too, unless there are extenuating circumstances.
cheers,

how much oil is loosing? a lot or just a little be? because belive it or not a little oil losse is normal on engines but not a lot.