Certainly saying nothing might be acceptable in cases where nobody needs to move, but an annoyingly large percentage of the time people are walking side-by-side taking up the entire trail with their backs to me, are on the wrong side of the trial, or are one the correct side of the trail but their dogs are on- (or worse, off-) leash on the opposite side of the trail. Then what? I try "excuse me," though it sounds peculiarly polite.