There is nothing that will guarantee more success, or higher gig stats, or even a specific place in the search results. Adding a good video to your gig will merely help prospective buyers to know and understand more about you and the service you offer. A better understanding of your service might lead to more sales, however, it will not guarantee them.
The only way to obtain more sales, is to be a great seller, who has a well-crafted gig, and someone who offers a service other people need. There are no guarantees; there is only action taken to pursue and reach the goals you set for your business.
Focus less on the idea of guarantees, and more on how you can best present your service.