The best way to prepare is to be extremely proficient on your instrument procedures and instrument skills. Many regionals use a PCATD, so if you can get some time using one of those (or a FRASCA or even full-flight simulator) that will be a big confidence booster for you.

If the cost is reasonable, I recommend doing any sim prep that is available. Having said that, I'm amazed that folks are willing to drop $500+ for a single hour of CRJ sim prep prior to interviewing with a regional that pays $25k or less the first year. I suppose, however, if that is THE job you want its a small price to pay for better odds, is it not?