Have been using opendns for dns content filtering for a few weeks now.
I highly recommend it. All you have to do is put their dns servers on your computer or router. You can get an account to get more fine grained control on what to filter.
Not intended to be foolproof, but does a good initial job of stopping me even accidentally drifting into seedy websites.
The other thing I would recommend to make your web experience less seedy, is to run an adblocker plugin to your web browser.