I've actually answered this question before in a post entitled “How To Optimize Your Sales Funnel“.
Before I hop into what I'm about to say … first and foremost … you need to be offering something people actually want. If what your offering … no one cares about … the rest of this podcast is all a moot point.
Anyway, to answer your question – you need to first think of your website as a system.
It's not just “one thing” – there are many, many, many moving parts and they're all connected. If one part is broken, it's going to impact other parts.
At the beginning of your system (or your website) – you have your traffic source … which is arguably the most important aspect in regards to how well your site will convert.
If your traffic is bad … it does not matter how great the rest of your system is … no system in the world can turn garbage into gold.
So, traffic source is the first “best way” to increase your conversion rate.
The second best way is through optimizing your landing page. If you're unfamiliar with that term – a landing page is simply the page someone “lands” on after clicking through to your site … whether through an ad, a search result, a social share, or whatever.
If your landing page doesn't deliver clear guidance, have solid copy, etc. It's going to hurt your conversion rates and it will need changes.
So, that's what I would look at next.
Thirdly, if you don't have one already … you will want to have email autoresponder series in place. And, you'll want to ensure it's optimized as well.
Those are the three things … in that order (traffic, landing page, email series) … I recommend enhancing/optimizing as the “best” ways to increase a website's conversion rate.