Good quality content - and building authority, especially with quality websites linking back to you.
Here’s a good article on google’s sandbox:
While Google sandbox is not something you can control, there are certain steps you can take in order to make the sandbox effect less destructive for your new site. As with many aspects of SEO, there are ethical and unethical tips and tricks and unethical tricks can get you additional penalties or a complete ban from Google, so think twice before resorting to them. The unethical approaches will not be discussed in this article because they don comply with our policy.
Before we delve into more detail about particular techniques to minimize sandbox damage, it is necessary to clarify the general rule: you cannot fight the sandbox. The only thing you can do is to adapt to it and patiently wait for time to pass. Any attempts to fool Google – starting from writing melodramatic letters to Google, to using “sandbox tools” to bypass the filter – can only make your situation worse. There are many initiatives you can take, while in the sandbox, for as example:
Actively gather content and good links – as time passes by, relevant and fresh content and good links will take you to the top. When getting links, have in mind that they need to be from trusted sources – like DMOZ, CNN, Fortune 500 sites, or other reputable places. Also, links from .edu, .gov, and .mil domains might help because these domains are usually exempt from the sandbox filter. Don’t get 500 links a month – this will kill your site! Instead, build links slowly and steadily.
Plan ahead– contrary to the general practice of launching a site when it is absolutely complete, launch a couple of pages, when you have them. This will start the clock and time will be running parallel to your site development efforts.
Buy old or expired domains – the sandbox effect is more serious for new sites on new domains, so if you buy old or expired domains and launch your new site there, you’ll experience less problems.
Host on a well- established host – another solution is to host your new site on a subdomain of a well-established host (however, free hosts are generally not a good idea in terms of SEO ranking). The sandbox effect is not so severe for new subdomains (unless the domain itself is blacklisted). You can also host the main site on a subdomain and on a separate domain host just some contents, linked with the main site. You can also use redirects from the subdomained site to the new one, although the effect of this practice is also questionable because it can also be viewed as an attempt to fool Google.
Concentrate on less popular keywords – the fact that your site is sandboxed does not mean that it is not indexed by Google at all. On the contrary, you could be able to top the search results from the very beginning! Looking like a contradiction with the rest of the article? Not at all! You could top the results for less popular keywords – sure, it is better than nothing. And while you wait to get to the top for the most lucrative keywords, you can discover that even less popular keywords are enough to keep the ball rolling, so you may want to make some optimization for them.
Rely more on non-Google ways to increase traffic – it is often reminded that Google is not the only search engine or marketing tool out there. So if you plan your SEO efforts to include other search engines, which either have no sandbox at all or the period of stay there is relatively short, this will also minimize the damages of the sandbox effect.