I like the analogy of our karma as a Bank Account and would like to take it a step further:
At birth - was are carrying our karma with us like a Balance Sheet (an accounting term)
Balance Sheet contain the following (in a simplified form):
(A) Long Term Assets - White karma that would ripen beyond this life time
(B) Current Assets - White karma that would ripen in this life time
(C) Long Term Liabilities - Black karma that would ripen beyond this life time
(D) Current Liabilities - Black karma that would ripen in this life time
Each of us were born with a NET balance of either black karma (A+B) < (C+D) or
white karma (A+B)>(C+D)
If we have a huge amount of B, we will have a very comfortable life - but the catch is this -> it will be depleted, by enjoying the result of white karma, we also using it like money in a bank !!! Hence if we don't accumulate white karma in this life time (even though we do not collect Black karma), the white karma will eventually be depleted and very soon, we will face rebirths with net black karma and suffering with begin, let alone progressing in spiritual path.
Conclusion - do good deeds, practice giving and generosity, collect white karma while we can, in all our lives from now on and at the same time avoid bad deeds, engage in dharma work and prayers to purify accumulated black karma - work on our karma balance sheet!!
I hope i did not confuse anyone :-p