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I know several people (mainly broke college kids) that have aging cars (read: beaters) that, while they have their share of issues, have motors that are on their last legs - losing compression, burning oil, maybe some smoking.
What are some ways they can extend the life of their motors or get more use from their car for the least amount of money?
Being a broke college kid, and having driven, broken and fixed my own beaters, I know what you're asking.
Keeping your oil and water systems in good shape is the single most important thing when driving a beater. A good investment, although it is often seen as overkill, would be replacing the water-, and oil pumps.
Spark plugs can go a very long way by just cleaning them every couple of thousand km's, or when the car starts having trouble starting, if you're lazy.
Air filters also respond well to a couple of slaps, to get rid of all the dust gathered on in. Unless you find it drenched in oil, this doesn't need replacing all that often. A clean filter also helps along fuel consumption.
Engines actually can last a lot longer than you might expect, even when burning oil and having bad compression. Just replace stuff as it stops working and hope you get enough financial resources to deal with it before everything fails in a big way. Spark plugs can last for years so long as they stay clean and undamaged. Air filters can be cleaned and reused. Never run super unleaded. Never use injector cleaners or any of those snake oil treatments they sell in stores. It's all bunk.
The main critical things to keep an eye on are coolant and oil levels. Without coolant, your engine overheats and this causes lots of expensive and annoying things to happen. Without oil, you can spin bearings and generally weld moving parts together from friction heating. This is also expensive. Coolant leaks are much worse than oil leaks. So long as the oil doesn't gush out and lose pressure, it's fine. With the coolant, a leak will let coolant boil at lower temperatures, which can cause your car to overheat because it's all boiling inside the head instead of carrying the heat away.
Piston rings and low compression are not an immediate problem. Modern oils are pretty good at soaking up blowby contamination before they lose the ability to lubricate, and being down on compression will waste a little gas and cause you to be generally down on power but this isn't going to constitute an existential threat to your engine. Just keep the oil topped up and change it regularly.
The main worn engine problem you're going to have is a side effect from severe oil burning. It's going to cause deposits on the backs of the exhaust valves, which will eventually cause you to burn a valve, at which point you are going to be down a cylinder (assuming the valve debris gets swept out the exhaust port and doesn't fall into the cylinder). Oil burning is usually caused by worn rings, worn valve guides, worn valve seals.
Once you burn a valve your two options are to a) replace the head (cheap) b) replace the head and rering the engine (expensive). Replacing the head can be accomplished over the weekend (with practice) and you can easily limp around burning oil for a few years before you have to swap in a fresh head. Since a 4 cylinder head can be completely refreshed for usually a couple hundred dollars, this is a pretty economical way to keep a beater running. You just swap in a junkyard head, get the other head repaired and in a few years swap it back in again.
Keep in mind that until you fix the rings, the oil burning and the smoke are going to be pretty disgusting. Also, burning oil kills catalytic converters and it's not really great for your hydrocarbon emissions either. People will hate you and you will feel ashamed for your sooty car. That being said, it's always better to be poor with a crappy car than poor with no car.
*Note that there are multiple types of smoke that can come from a car. If you have a blown head gasket, that is different than smoke from burning oil. You need to fix a head gasket if it goes.
All of the issues you describe are physical damage or wear to the internal engine components. There is nothing you can do to make it last longer. No oils, additives, or witchcraft will help. Physical repair of the issue is the only thing that will help.
Sorry, but that's the bottom line. Perhaps you should learn how to work on cars. I suggest going to the library, there are many books to can check out (for free even!) on auto mechanics. The internet is not the best place to find this information. It is full of uninformed opinions and unqualified experts hiding behind their keyboards.
