Sorry for late reply. Was kinda busy. Don't know whether this reply is still relevant or not. However, unanswered questions don't look good, so here you go.
As far as trick is concerned, reading RPGLE is a different thing and practising it is altogether different ball game. So, according to me, the trick lies in practice. One problem with practice is that AS400 systems are not easily accessible. For this, you can get a free account at
http://www.rzkh.de/english/indexeng.html. This is a very old website providing free as well as paid access.
Then you require some good basic material. What I referred was the material provided by my company. Lateron I referred RPG IV by Robert Cozzi. Really good book.
If you have any specific question, do let us know.
Happy learning!
