Here’s a flash game that’s not only fun, it’s good practice. Park your car four times – once going forward, once backward, and twice parallel (once without any obstacles in the way, and once with a dog in the way). I was trying for speed at first, but if you crash in any of the four levels, you have to start all over again, so I slowed down and got a 77.1. I believe lower numbers are better.
Aim more for precision and less for speed, at least on the first few runs so you get a feel for it. Also, apply the brake (spacebar) liberally. Even if you ever so gently nudge a car (which I did on numerous occasions), it’s considered a crash and you go all the way back to the start.
If you want even more of a challenge, try Parking Perfection 2 and Parking Perfection 3 : The Exam.