I purchase these
Clorox toilet bowl cleaning tablets, you can drop one into the toilet's tank and it'll last about two months. It gradually releases into the water while it sits in the tank, and keeps the toilet bowl clean with every flush. It's especially nice if your toilet bowl is prone to develop discoloration at the water line.
Things to note:
* This does a great job of keeping the toilet bowl clean, but if it's already dirty, you'll want to use toilet cleaner with a brush for a deep cleaning first
* You shouldn't use these with toilets which aren't flushed daily (eg, don't use it for the guest bathroom that doesn't get much use)
* Some people claim that these can corrode bolts inside your toilet tank. I haven't done much research on this, and I don't know if it has merit or if the corrosion people are seeing is due to old toilets / other causes / incorrect use
* Your restroom will have a faint smell of bleach - this is the cost of convenience with these