Yelp gives value to negative reviews much. If any fake user post negative and positive reviews. You can see his negative reviews is their, but positive reviews is removed or not displaying.
To do a successful yelp campaign, do below things
1) Connect your facebook profile with your yelp.
2) Post reviews only in a specified city limit i.e if you are posting reviews in bar in Houston, then all reviews from that profile should be posted in Houston area business only.
3) Create a friend base in yelp for each profile.
4) Never make negative and positive reviews in two competitor business from same profile.
5) Make sure all your reviews should look like professional review.
6) Add same facebook profile picture to your yelp account.