
There is the optimization you do on page by studying your competition, learn from the other freelancers' text and always improve your gigs. And there is optimization off-page, by providing backlinks to your gigs. Don't buy those backlinks, add them in a natural way by writing about the services you are offering on your blog or on social media (the tweets for Twitter are index, the Facebook pages are also crawled by bots). You can post on social media either your gigs or your blog posts to attract more people on Fiverr.
Upload some videos on YouTube. By talking about about your services you let the potential buyers descover you and your gigs using one of the most popular search engines on the internet (because Youtube has the value of a search engine, with algorithms similar to Google).
You should also use the referal link you get from Fiverr. If your public is not on Fiverr yet you can get some extra money by convincing them to join.