The current method to control the bitcoin supply to a maximum of 21 million BTC is to reduce the block reward (currently 50 BTC/block) by 50% every 210000 blocks. In this forum post there is a discussion about "why doesn't the block reward decrease continuously?".
The OP writes:
So I personally wish that Satoshi had instead implemented a continuous block reward decrease. With that approach, the reward will decrease ever-so-slightly with each successive block but still converge to 21 million. That would avoid potentially disruptive discontinuities. It's too late to change that now but, still, does anyone know why he chose a block reward function that looks like a staircase rather than a smooth curve?
This gets rid of the discontinuity (block reward going from 50 to 25 BTC) at the 210000 block mark and at each subsequent block reward readjustment. Would this be better than the current method? Are there other advantages to this method? Are there any disadvantages to a continuously decreasing block reward?
One disadvantage I can possibly think of is that this would favor "early adopters" even more than the current bitcoin implementation.