TPK,
Trevor Thom #5 was my study material and got me over 90% in the exam. However, putting the exam to one side, I feel that it gave me an excellent knowledge base for my IMC training.
A couple of other resources to consider,
www.Airquiz.com offers online practice IMC papers for the paltry yearly subscription of £3. Your attempts are automatically marked and you get emailed the answers for the ones you get wrong within about a minute.
The magazine "Flyer" has a monthly instrument teaser.
FS2002/2004 can help for practicing the actual flying, particularly NDB tracking and approaches.
IO540's advice is good, because one of the main reasons for having the IMCR is to get somewhere when it's grotty. Hence, me and my instructor have been doing just that, going places during the course (namely East Mids, Leicester and Cranfield), practicing diversions, using class D airspace and major airports. The flying syllabus is not as closely monitored or directed as the PPL, not having a list of exercises for the student and instructor to stick rigidly to and allowing quite a bit of flexibility and interpretation. Mind you, I wouldn't have thought that many instrument flying instructors exist who don't use their privelidges in anger.
Regards
Obs cop
Ps. I have a copy of #5 but it's mine, all mine I tell ya.