boarding mid-itinerary
 

Is it possible to board the second segment of an itnerary without flying the first? For example, I'm booked from St. Louis to Cleveland to Denver. Can I skip the St. Louis-Cleveland segment and get on in Cleveland?

No - they will cancel your itinerary if you don't board the plane in St. Louis.

Although, if you checked in online & the flights were different flight numbers & different planes, you might be able to get away with it - I wouldn't intentionally try it, unless you've got a long connection time between flights in CLE & you can say you missed the flight from STL-CLE, so you bought a full-fare on another carrier to get to CLE to finish your itin - however, I doubt that'd truly work out.

Way, way back, some 10 years ago, you could have them print all your boarding passes in advance and just walk on the plane (or not) as desired, but those days are gone. The only scenario I'd try is if you had all carry-ons and could fly every segment except the last one.

No way. You can get off after the first segment (with carry ons) , but not get on the second.

Miss a segment and your ticket becomes worthless from that point on. Show up to check in and if they realize you booked it to save a ton of bucks, they may tell you can board, if you pay the extra money.

Actually, if you have a good reason (other
than fare-dodging) to do so, you could ask
the airline nicely, and they might let you.
Get it all squared away first by telephone,
of course.

same question?
 

Is this any less of a problem if you want to skip the first segment of a return flight and get on at the connection city?

No, same rules apply. If you skip a segment, all subsequent sements are cancelled. As posted earlier, the only one you can even think about doing this is the final segment, and even that might be tricky (against the rules, but you might get away with it a time or two).

How unfortunate (for me). I didn't realize this was the case and booked a flight a couple of days ago that is going to be very difficult for me to get on the first leg of. Blecchh.