Keep all clauses of a predicate in the one place. Leave at least one blank line between the clauses of one predicate and the next (particularly if they have similar names), since otherwise they may at first glance appear to be all from the same predicate (of course, this never happens because there's a comment before every predicate, right?). Similarly, it is best not to leave blank lines between the clauses of a predicate (particularly if done inconsistently), since otherwise at first glance they may appear to be from different predicates.
Clause heads should be flush against the left margin. As well as making them easier to pick out visually, this makes it easier to grep for the definitions of predicates (as opposed to their invocations). The head/body separator `:-' should follow the head on the same line. The body of the clause should then commence on the next line, indented one tab stop.
non_overlap(Start1, Dur1, Start2, _Dur2):-
Start1 + Dur1 #=< Start2.
non_overlap(Start1, _Dur1, Start2, Dur2):-
Start1 #>= Start2 + Dur2.