The problem is that you don't really think of them as having different plumage in the fall as the spring, but they change enough to be troublesome. The Common is on the left above and the Forster's on the right. When I'm up close, the shorter legs and trimmer build of Common stands out. At the right angle the darker mantle of Common is helpful, but at a lot of sun angles there's not a ton of difference even with direct comparison. With direct comparison the redder bill and legs of Common is frequently a shade darker in the field than the orange of Forster's
Here's a Forster's showing another problem with fall ID:
This bird, a little farther along in moulting (as evidenced by the cap) has the tail at the proper length.
It varies though, upon both individuals and the angle.
Moving to Common, they also have darker wings than they exhibit in spring, quite blackish when folded.
The primaries aren't as dark in the spread wing than as when folded, which doesn't make it easier to tell them apart.
Finally the one-year old Common's that are present.
The dark carpal bar (which juvies will also show when they arrive) is a dead giveaway that it's a Common.
The (quite faded) one-year-olds are quite pale in flight.
The other ID difference you may have noticed is that a portion of the Common Terns are banded, I didn't see any banded Forster's.
Speaking of which, I added a comment from one of the plover researchers, one of the Piping Plover juvies is from Ludington and one from Sleeping Bear.