One day, a classmate of mine at film school read the script and really liked it. She wanted to produce it so I gave her a free option. She began to call these agents and told them that she was a producer (sort of true) and that she had optioned the script (also true), but that she wanted to get the writer an agent before she sold it to the studios (a bit of a stretch). Every agent she spoke to read it that weekend!
Of course, I never signed with any of those agents, so the trick only works so well, but the rule of thumb makes sense: Someone (anyone) else saying your script is great is infinitely better than you doing it yourself.
We eventually got an agent to represent me by first getting an attorney on board. Attorneys (or managers) are easier to get and can give you that key referral the top agencies always say they want.