Above all, be clear on what you're looking to get out of it.
The New York Jets came to me in 2009, just as I started VaynerMedia, because they wanted to sell me a skybox in the stadium for Wine Library.
I told them, "Forget about the skybox. The Jets need to be at the forefront of social media." They were unsure about hiring me, so I said, "Give me one player's Twitter account, and I'll show you what I can do for free."
I knew it was my cost of entry. Working with the Jets is how I was able to land the Nets and the NHL as clients, too.
People overthink spec work. They get too philosophical and romantic about it. I'm not romantic about anything in business. Don't be crippled by the philosophy.
Aim for the biggest possible client you can get, and treat it as if it is the highest-paying client of all time. If, in the end, they try to take advantage of you and get more work for free, remember: You can fire them as easily as they can fire you. The reason you're doing all this is to get other clients, period.