A lot has changed over the last few years since my colleagues and 500 founders last commented!
While we value the referrals and dealflow of our mentors / founders / co-investors, what is most important is YOU and your STARTUP! This is what we are currently looking for as of Dec 2015:
-Complete and launched product
-Balanced team (hacker - someone who can build, hustler - someone who can sell)
-Good unit metrics (what is your CAC, LTV, retention / engagement? If you don't know these terms, you should look them up!)
-Some semblance of revenue / usage (i.e. enough data points for your unit metrics) This level is typically between $10k - $100k MRR, though this is not a hard and fast rule. These days, we are focused on helping late-stage seed companies grow during our program, and we are looking for companies with product-market fit / are near there. We have a distribution team (on our payroll at 500 Startups) who help our companies optimize their customer acquisition processes during our program, so you must be far enough along for us to be able to help. This is a change from a few years ago, when we were accepting super early companies.
To best increase your chances, you should focus on your unit metrics. Firstly, you should know what they are. Secondly, if they are "bad", you should work on optimizing them. That said, we do understand this is a work in progress AND unit economics change over time. So, at the very least, you should be able to discuss what experiments you are doing to decrease your customer acquisition costs, increase your retention / LTV and what levers you think are most important. We are not simply looking for numbers but a smart conversation about your numbers.
Lastly, because our program is so focused on growth, we will want to know the details of how you are acquiring your customers. You should be able to speak to this IN DETAIL. In addition, we will ask you about experiments you are running / want to run to acquire customers, and you should think through this and convey this in both your application and interview.