...I don't do it myself, but all the books recommend it. Suppose you are trading intraday on a one minute chart and you have 2 years of data to work with. The rule of thumb is to divide that into ten parts. Optimize on the earliest tenth of the data. Then run the optimized system on the next tenth only and see how it works. Then reoptimize on both tenths, and see how that works on the third tenth. Repeat until finished.
The idea, which I think is stupid, is to get a feel for how it would have traded, and for how robust it is over changing conditions. I prefer to get a wild assed idea, BT it for 60 days, watch its performance for a month, trade it if it's good, and reoptimize it weekly to keep up with current conditions.