Bruce Snyder's book Save the Cat offers a simple construction pattern to follow when building a logline. I was taught to work off my script's theme, a good practice. But the logline is more concrete, tangible. Similar to a thesis statement in an exposition paper. It's a single sentence - two, if necessary, but in the words of Walter White/Heisenberg, "Tread lightly"- that contains: the Who and what the who Wants, with implied conflict. Remember: only one sentence, this makes the logline easy to memorize and makes you and, thus, want to take it with you. Like a fine piece of luggage.
Our class had loglines that traveled from talking extraterrestrial cats, to corrupt cops, to long-distance lovers, to immigrant gravediggers, to Russian Jewish folklore.
In the end, they were all toting some respectable loglines.