I was skiing about a month ago with a guy who has a business that does exactly what you are looking for, he helps businesses improve their Yelp reviews. I'll get his info and pass it on to you. Apparently his is a very legit service and guarantees success, etc...
The one thing that he told me over lunch is that you never, ever want to try to game Yelp by having friends or some bogus service post phony reviews or the like. Yelp has a very sophisticated software to detect that and when they catch you, (which they will), you will get a huge banner across the top of your page stating that they detected fraudulent reviews on your site.
I know that you have not suggested such a thing but others here have and it's the worst advice possible, as has been noted. There are legitimate ways to sometimes get negative reviews removed, that is what my acquaintance's service does, among other things. They also help you improve your score on Yelp in other, (non-fraudulent), ways.
