Depends on a whole bunch of factors. Do you have money to spend on online ads? Do you have people in house to build growth features? How niche is your website? Are there existing distribution platforms you can launch on? The following strategies have worked for me well depending on the type of product:

1. Building viral growth features directly into my site to amplify traffic (lots of potential here depending on your product/site)
2. Spending on online ads on Google, Facebook, etc. (if you want this to be sustainable, lifetime value should be significantly more than cost of customer acquisition, but this might not matter in the short term)
3. Generating PR (hard and short-term but when it works, it works)
4. Striking partnerships with other high volume sites OR getting on a platform that gives you immediate organic distribution. If you can get promoted by the platform, that can REALLY help.
5. Content marketing, SEO, etc. are slower but can be important in the long run

I've typically found that one strategy works best for any particular site so it's important to iterate until you find the best one.