I know some of you say, 'F-man, you elitist, pretentious bastid, not everybody can afford to buy a new car. Some people have to by cheap cars because it's all they have to spend'.
If you're gonna spend money on a high-mileage and/or old car, fine. Have your mechanic look at it and make sure it's safe and won't break down next week. Thing is, don't sink money into it. If something breaks that costs say . . . more than $100 to fix, get rid of it and get another cheap shit to drive until it breaks. Oil changes and wear items (batteries, brakes, belts, bulbs, etc don't count).
Rule of thumb: Never put more money into a car than it's worth. If your mechanic says it needs a timing belt or a water pump and it'll cost $500, tell him to take the license plates off it and junk the thing. Ask him to find you antoehr cheap pice of shit to run until it dies.
Recognize the $800 car for what it is, temporary transportation. The only people who should really consider fixing cheap cars are folks who can do the work themselves. (Example, my old Hyundai Excel. I got it for nothing but the head was so warped I had to do a new head gasket every couple months. It was $16 for the gasket and it got to where it took me 20 minutes to change. I did nothing else to the car and threw it away after a couple years). Never, never buy one because you love the thing. Buy it because it starts and runs. At that point it doesn't matter what make or model it is, you just want it to run. When it stops running, get rid of it.