To start I think you already know the basic SEO and other methods about white hat. For starters, Fluctuations in rankings are normal. Your website can drop or go higher in rankings depending on how stable and strong your on-page and off-page optimization. Sometimes some small changes to your website can affect greatly or none.
All you can do is to build more links. Links which are strong enough to stable your rankings or just drop a little. Doing SEO should be continuous. Earning trust from different websites also boost your rankings in some ways. So just keep doing SEO for the site.