DEFINITELY seconding kakaku for a general search of ISPs, but in my experience, it's largely down to what campaigns are on and for how long. Some offer half price for X amount of months with a one month cancellation charge at any time. Ergo, if you have an insanely cheap deal, you could pay the cancellation fee and still have more money in your pocket than if you'd stayed the full amount.
Some companies, such as a J:COM, offer a package deal with cable TV and phone line, and these can sometimes be cheaper than a straightforward internet plan.
Also, if you net use is generally low, you could be incredibly savvy and use one of the many MVNO Data Sims in a 4G phone and tether it to your computer. Some MVNO's actually include WiFi deals in their package for use at home. This would make you totally wireless! (Might want to keep the phone plugged in to your charger though!)