1. Write a script that is so good that it will sell for big bucks.
2. Sell that script to a big studio.
3. Call an agent and say "I've got a deal with Warner to buy my script for $3 million, and I need an agent; do you want a piece of this?"
I've known people that get an agent after winning a screeeplay writing contest.
But usually it's what's above. You can't get an agent until you sell a script and you can't sell a script until you have an agent. That's why I've taken up beach combing as a career alternative.
Industry referral is the way to go. But by that, it means you have to know someone who's not only in the biz, but read your material and will vouch that it's that good.
Other than that simply write an email to an agent with a logline and hope they will request your SP and then rep you.
Going the manager route isn't such a bad idea, and can possibly save you the 10% of an agent (but managers are 10% too).