I'd like to add that I've found a couple interesting/useful alternatives to simply throwing them off the side:
Glide tossing, glide tossing, glide tossing. Not only can you glide toss them forward when you'd otherwise be out of throwing range, you can also glide toss them straight up. I often take the first one I pick up immediately after the match starts (assuming this is on FD), glide toss forward, roll back to pick up another, glide toss forward or up, more often up, since they're usually in the "oh hey I'm smart, I'll just jump over while he throws his crap" mode.
Glide tossing up will either : A) hit them and allow you to pretty easily catch the piece for another throw, or B) force them to stay back more than they initially intended, to which you can either catch the falling piece, throw in some whip spam, and/or run back and pick up the 3rd piece to throw again, usually with a forward glide toss.
Biggest part of this: People are far less likely to advance on you with items in the air that could fall on their head. Use that to make them keep their distance a bit.
Also it seems a lot of players get overly obvious if they manage to catch a piece to throw back. They're not typically very tricky, and very rarely know to/use glide tossing unless they're playing a character that has similarly functioning projectiles already (Peach, Diddy), so if they do catch one, just shield often until you get in a position to hit them with aerials or Side B's.
