I just want us to share our opinion based on the experience of promoting/optimizing Fiverr gigs - in ways which really worked, not only theory.
I will start first and share 3 ideas which I have used.
1. Get a big number of sales in a short period (1-2 days) and over deliver. This helped me get a better positioning in Fiverr's SERPs
2. Using the main keyword of your gig in the first paragraph. This boosted my rankings quite fast - and I've got on the first page in less than 24h
3. Never lose a review. Once an order is marked as complete automatically, I contact the buyer and offer him a bonus in exchange of the review. 50% of them are willing to do that. It's important to do this every day - don't delay this - the fresher the order, the biggest chance to get a review.
I hope these are useful and also that everybody will start sharing some useful inputs - I'm more interested in promoting the gigs outside Fiverr - but I will appreciate any of your ideas and hopefully everybody will have something to earn from this