Also in preperation for the IR and have found RANT XL to be very useful.
It allows you get the fundamentals right before going anywhere near a very expensive twin-engined aeroplane. It has a series of tutorials on such things as instruments (HSI, RMI, EFIS etc), Navaids (ILS, DME etc), approach plates, radial intercepts - it goes on. Then exercises to consolidate your learning. I got mine for about £80, which of course is negligible compared to the IR course.
I've also got the OAT CD-ROM for IMC to go over the basics, and a pretty good book called (funnily enough) 'Instrument Flying' by D. Hoy.
It prepares the pilot to pass the 'most demanding flight test in the world' and covers navaids, let-down and approach procedures, airways flight and how to pass the test itself.
I think you can't be too prepared for it.