Every time I deposit a group of cheques, I write out one to the receiver general for 18.1% of whatever it is I just cashed (18.1% is the combined federal and provincial corporate tax rate where I live).
Keeps taxes 100% up to date, and then I get a refund for whatever expenses I incurred, roll that into the following year's taxes. This year, I didn't have to start writing cheques until august due to last year's refund. All I have to worry about then is personal tax at the end of the year, which usually is pretty (and this year will be totally blown away by rrsp contributions).
