first: your top/left/right properties are sort of wasted here (you must define the position property to be able to use them) -- and you have an extra bracket before #form_rightside that could be causing some sort of problem (but that could simply be because of bad copy/pasting). but other than that, everything looks like it would work.

you really probably want to post your mark-up so that we can see some context, along with the rest of the stylesheet as well. if you're clearing on the right, you obviously have some other elements floating around and those elements are probably making your stylesheet not work the way it should.