Its amazing how much FS2004 and the like can help with a sim session. F2004 will not teach you how a plane handles etc but it will get you up to speed for procedures and call outs etc Treat it as a procedure trainer and IMHO you can not go far wrong.
I have only had one sim check so far, however I passed and have proceeded onto the final recruitment stage. I would suggest not only focussing on your flying, but also what you can do on top of the flying, answering ATPL type questions, eta's and things like this. The flying has to be second nature and you should be left (even accounting for nerves) spare capacity to deal with other issues.
I don't think they look for perfection, just the ability to improve and they (in most cases) want you to pass and will not try to catch you out.
I was lucky in the fact that my sim check was quite gentle, SID, VOR tracking, general handling, VOR/DME app, radar ILS-go around, radar ILS. I had to answer questions whilst I was flying, but he was only asking relevant questions, eta's etc .It was a brand new FNPT2 which could be kitted out as any one of a number of aircraft. They had chosen a Kingair for my session, I had been told to prepare for a completely different sim and aircraft so it was quite a surprise for me
My advice is to relax and enjoy the experience, treat it as a learning curve if things do go wrong.