If you are serious about your website you will have installed some kind of analytics software so you can monitor what's working and what's not. Google Analytics is a good and free option.
If you look through the keywords section you will find keywords and phrases that have sent traffic to your site. Some of these will be phrases you have chased but many of them will be keywords and phrases you haven't.
Find a keyword that's sent some traffic your way and see where your site is placed in Google for that search. If it's languishing way down yet it still sent some traffic your way imagine what it might achieve if you optimise a page for it!