I found that the more simple you keep things, the better. I played levels 1-40 when the game came out and managed to get expert. Tonight I went back to Parking Dash and decided to beat levels 41-50 and get all expert. I was suprised but I managed to do it- The first Dash game I have achieved all expert on. Below is my advice more specifically on levels 41-50.
 
*You do not need to use the nitro booster at all.
*You do not need to swap any cars at all.
*Make sure to keep making chains where you park the same color cars in the same spots as much as possible.
*Try to park the guy with the dog isn the back- It is a real pain to move him.
 
The customer that you need to watch the most is the limo dude. He gets angry the quickest. Also, you need to pay close attention to the cell phone dude and African American woman. Make sure you serve these customers right away or they will turn red and you will end up losing money from them.
 
I found levels 44-50 a lot easier than 41-43 because you were able to give the customers the sodas to cheer them up. Make sure to use this power up as frequently as you get it!

hey thanks to your hints i beat the whole game and got expert 41-50---thanks for the help

Thank you so much for your tips! I had been struggling with level 43 for the longest time. I had already given up on this game, but I tried it again and it was so easy to get expert on it. I think I'll go back through the previous levels and try to get expert on everything too. It's a lot easier once you get past 43! :)