Pay attention also to your code. Instead of executing a dead loop for delays, use a sleep mode of the processor thus reducing the current consumption significantly and maybe just a few mA-mps would do. If using only 89C2051, reduce also the supply voltage to 3V, and your power source can be even smaller.
If you need a delay say 0.5 second, it is a good practice to run a timer interrupt in autoreload mode at a rate of 1ms or 0.5ms or 0.25 ms (depends on the crystal you are using). In the delay routine put CPU into sleep mode, when timer interrupt happens, CPU wakes up, then decrement timer delay value by 1 and go to sleep again, until the timeout counter is 0. This way, CPU is sleeping most of the time and the power consumption is reduced significantly.