From my MCS BASIC52 book from Intel, their solution is:

xch a,dpl
jnz decdp
dec dph
decdp:
dec a
xch a,dpl

And it doesn't trash the Acc!

this can also be modified to decrement other variables apart from the dptr. Many of the 'c' compilers implement the unsigned int decrement the same way.

Another method is to subtract 1 (or add -1 ). A little more obvious but I doubt any more efficient.