What about what not to do? Well, these are some of the common mistakes you're likely to make when first starting to craft a logline.
Don't use character names
It's tempting, but don't do it. The goal here is simplicity and brevity. So, rather than saying. 'John is a doctor whose wife has just left him', try 'a recently divorced doctor...' The fact he's 'John' is of no importance at this point.
The only time you could waive this golden rule is if you're writing about a real life human being.
Don't try to tell the story
It's only one sentence, it's just not possible, so don't even try.
Don't be tempted to use analogies.
Your screenplay might be Rosencrantz and Guildenstern are Dead meets Mrs. Doubtfire, but that kind of comparison is not useful for a logline. Remember, you've only got one sentence. Not a single syllable can be wasted.
Don't use hyperbole
'Greatest story ever told'...'A masterpiece'...'Heartbreaking and beautiful'. You know, the kind of thing you'd find on a movie poster. It's really not for you to decide whether or not it's the greatest story ever told, and, let's face it, chances are, it ain't. So leave it out.
Don't confuse a logline for a tagline
A tagline is also a one sentence hook for a screenplay, but it's usually for movie posters as a marketing tool.
The logline is a marketing tool too, of course, but directed at film makers not film goers. Taglines are much more succinct and usually a little more obscure; they don't give the important elements of a screenplay. For instance, one of the taglines for Beetlejuice is, "He's guaranteed to put some life in your afterlife."