If filing Married filing seperatly, how do my husband and I decide who claims what?

I have done both of our taxes jointly and seperately and it would benefit us both to file as seperate. My question is, who claims the mortgage and other deductions if both of our names are on the forms? Can we both claim them or do we have to split it? I have an appointment with H&R Block but I don't want to pay them if I can do our taxes myself and this is the only thing I'm stuck on. Any help would be greatly appreciated.