Basic painting guide needed

[BradZ]

After seeing many forum members painting interior bits and also engine parts i've always wondered how the turn out so good. I've painted the odd engine cover and other misc things but they never turn out as good as some members on here. If anyone can please point out what I'm doing wrong it would be greatly appreciated. Seeing as i only really paint plastic I'll explain how i did the engine cover.
Pulled it off
Lightly sanded it-unsure of what grade
One coat of plastic primer
Spray with VHT Engine Enamel, yellow
Apply 2nd coat untill coverage is even
One coat of clear coat.

Obviously I am doing something wrong because it came out really flat. Even coverage, but zero shine to it and scratches very easily.
Again, anyone who could point me in the right direction would be great.