Yesterday my app went one ranking up.you can see the results below. Google takes everything into account while ranking. uninstallations , bad reviews , crashes etc. I agree that you can get more buy real app installs if you market your app. but you can retain your users only the the app is good enough and has more features.if you just published app work on the features not on the downloads.once you fully finished start android promotion in blogs , Google plus , send invites to your friends make them review your app mention your app wherever you can. but give utmost importance to the functionality. Do app store optimization when you get bored or in your free time. Let your app promote itself. Just fuel your app with features.
I did lot of posts about my app but i didn't get much downloads and no improvements as much as my expectations.I was focusing on the features I did lot of improvements in my app. I quickly responded to my feature requests. On the first month of publishing google will let your app to show on top.I don't know how that thing works. After a month my app went down in search results and slowly climbing up in search results. Whenever i climb up i get more downloads.
The reviews you buy from all the buy app installs and reviews services are deleted in a few days, most probably because they are done either from fake/duplicate accounts from the same device or something of a similar shady nature. In my experience, it has been a waste of time and money. It is a similar story for Facebook 'likes'.Look out for some developer forums and groups online, you'll find many. As Jay Paek mentioned in his answer, you can find several opportunities to exchange reviews - scratch my back and I'll scratch yours. :D
Apart from that, get your friends to download and write a review in a few words (try and get important keywords in their reviews). Trust me, this is one of the litmus tests to test your friendship - a true friend will always download and rate/review your app, and some people who you consider as 'good' friends will ignore your pleas for help :P
Try and get your app users to like your Facebook page/Twitter handle, and keep these channels active with periodic interesting posts. Eventually, you will have quite a following on these platforms, so when you launch your next app, you have a platform with your existing users ready to use for promotion.