1. Clearly define your risk profile
The definition of a 'startup' is nebulous at best and can mean completely different things to different people. Some think of startups as a couple scrappy founders in a garage who are eating ramen noodles. For others who come from a Fortune 500 background, a startup may be a venture backed company with 100 employees that has raised $20 million.
It's important to hone in on your exact risk tolerance, as this will allow for segmentation of the broader startup universe and allow you to hone in on specific targets. Some people are ready to take the plunge as a first business hire in a 2 man startup with no revenues; others won't be comfortable until there is revenue traction, VC funding, or both.
What's important to note here is that there is no inherently better stage of company. Risk tolerance is a highly personal decision that depends on a number of factors including age, family situation, financial obligations and personality. The key is to figure out what this means for you and then target companies in this profile.
2. Get passionate about something in particular
Once you've figured out what your risk profile is, the next step is to find companies that might be a fit for you. I find that too many people are interested in 'working for a startup' as a concept. If I'm honest, this is exactly where I was in 2006 when I was trying to land a job at a startup. I think I may have even said, "I don't really care what kind of startup I work for, I just want to work for a startup."
In retrospect, this line of thinking was way off. The novelty of startup life wears off pretty quickly, especially if you're not in a company that's growing like a rocket ship. My advice is to find a technology or specific industry that you are passionate about and then try to consume as much as you can within that space. Become an expert, begin to interact publicly on twitter and at meetups with people in that space. Figure out if you could obsess over trying to build a company to win, because that's what it's going to take!
3. Get out and meet people
The absolute best thing you can do to find a spot in a startup is to get out from behind your computer screen or smartphone and meet people face to face who are involved with startups. With even a cursory glance at a Twitter stream, you can identify a few interesting, connected people within your local startup market. Figure out a way to connect with these folks. If possible, a personal referral is best but don't be afraid to tweet them or send an email. Ask for a coffee meeting and a chance to learn. Folks in the startup world are incredibly open to helping others, as most have been on the receiving end of help themselves.
During the course of the coffee conversation, make sure to ask for a name or two of other people you should meet and, if appropriate, an introduction. If you're young and hungry, there may be opportunities to offer to do some work as an intern or for free. At a minimum, a simple thank you after the meeting goes a long way towards cementing the relationship.
If you get out and actively build your network, you'll be surprised how quickly the first 2-3 coffee meetings lead to a web of 20+ connections. As your network grows, you will soon be able to serve as a connector for others, adding value to the ecosystem and building your personal brand.
Working for a startup can be a fun, extremely rewarding opportunity. But it takes a bit of a process and a little work to find the right fit. Hopefully these simple suggestions will help you find the right company for you.