Don't waste your money on Drain-O or any of its similar counterparts if your drains are slowing down. Not only do they require you to use almost a whole bottle per "cleaning", but they are so bad for the environment. Instead you should grab yourself a handy long strip of plastic with barbs on the sides -- the Zip It. I found mine at Walmart and it was only $3. After buying it and using it in my old apartment several times, it got thrown out when we moved. The strip is durable enough to withstand many uses if you clean it off.
It took me forever to find it again because it seems like only certain Walmarts in our area carry it. But having resorted to several jugs of drain cleaner in between, I can definitely say the Zip It is superior.