If your rank is low, release, and release often. Don't sweat the reviews (use them as a means to gain feedback from the people who've downloaded the game). You can also make your app free for a couple of days which will bring in new eyes (you lose your rank if you do that, so just be aware).
As for being a top ranked app (and in my case, a top ranked app that lives and dies by its reviews), I really can't release a new version unless I add something that significantly increases end user engagement.
For example: v1.5 of the game hit the number one spot, and dropped to #2 18 days later. The moment it dropped I released v1.6 of the game which added developer commentary from me and Michael. We went back to #1 for 2 days after a week long slump. That being said, I compared numbers after the fact and our review conversion rates only increased by ~0.2%.
Full write up here: http://amirrajan.net/a-dark-room/#secondwind
Also, any tips on recovering from a ranking hit after releasing a new version and losing all your current version ratings?