In reply to tomaspkr:
I'd say the boots are too small but if you are sure it's not that, try lacing differently for the descent.
Most MTN boots have 4 "open" eyelets (2 rows) at the top of the lacing system above the locking eyelet. When lacing, if you go straight to the top row then work down, it helps push the heel into the heel socket and prevent rubbing on the underside of your foot. I say helps. It doesn't always work for everyone but it's definitely worth a try.
A suitable aftermarket insole is a must too. nearly every MTN boot is designed to have one instead of theirs (hence why they're always crap)