Brush up on Perf A, read 'Handling the Big Jets' and find a book on aircraft systems for complex types.
The first big surprise on an airliner type rating course is the complexity of the systems compared to smaller aircraft. Its a big feature of the job.
On the B737 you are always diving into the performance manuals as you often operate in the corner of the envelope. Knowing your basic Perf A will be invaluable here.
Check out the following website:
http://www.chris.brady.ukgateway.net/
This is for B737 guru's and had some good stuff on it. Glance at the MEL (minimum equipment list) and you will get a handle on the level of understanding of the job.
Finally, when you get to the course clear down all social activites for the duration and work really hard. Show a good attitude and, if you then have a problem, your instructors will be only too glad to help you through.
Hope this helps.
Things I wish I had known . . .