Look at what specific YC companies do, so you can infer what they might need... Lots of founders have blogs, Twitter, etc, and will speak their minds up out there. If you think you can create your own startup, you probably can do something of value to another, specially early stage ones... It can be as simple being an user of one of their products and offering feedback. I personally looked at YC company lists, saw ones that were on a similar space as mine, and, since I am from another country (Brazil), offered help. I ended up knowing a couple of YC founders that way.

Just remember to give first and always be nice... Startup founders usually have their hands full, won't always be able to help or even reply to you. And be resonable, don't think Drew Houston will become your best friend and take a bullet for you just because you sent him an email saying you like Dropbox...