There is always one surefire way to beat Marth (unless you're Fox).
Regardless of which character you are, you can always beat Marth by baiting out a move that isn't one of the follwing: jab, up tilt, down tilt, grab, neutral air.
So, by approaching just enough to get into his range and then switching up to a defensive stance, you can start to coax moves out of Marth. The big one you wanna look for is forward smash. Once he tosses that out, it should be a free combo, to be perfectly honest. And since Yoshi has his super cool DJ, he can absorb forward airs, too, which is the one move many other characters fear. If you can not only neutralize his forward air, but also punish him for using it, you'll do the equivalent of consistently powershielding a Falco's lasers. They've lost a means of approach...or, at least, they'll be wary when trying to use it.
And Bringer always says dash attack in this matchup, and I think he's right. As long as Marth's in the air, dash attack is almost always a sure thing. The least you'll do is trade, and that's never a bad thing in this matchup. Yoshi's survivability greatly overshadows that of Marth, so you can use the pressure of trading blows as a tactic to get into his head.
If he's trying to put ground pressure on you, lightshield. I was actually
actively using lightshielding in tourney today and it helped me out a great deal. It's a good way to reset and get yourself some breathing room, but you guys already knew that. ^_^
P.S. - Down air combos into forward smash tip at 60%. It's pretty gay. Look out for it. Don't get hit.
P.P.S. - Forward smash is your friend. I've won tourney matches against Marth using only forward smash and down tilt. It's pretty amazing.