I do a few different types of shield Pressure:
Eggroll, although punishable, does enormous shield pressure. If you use it creatively (ex. starting right below a platform where you would usually waveland so that you end up on the platform, then reversing and rolling off so that you have a hitbox in the air.) you can get in on most situations. If they shield, you can get out of eggroll and DJC N-air or F-air. If they shield that, you can L-cancel and jab. There's a thing I discovered where if you time your jabs, you can apply massive shield pressure and force them to make 1 of 2 decisions. What you want to do is to jab - jab - jab - jab in a rhythm. Doing a double jab quickly does alittle, but it's punishable. If you time the jabs in a rhythm, they get very limited options, and it also plays mind games on them since you can use the double jab in other scenarios. But if you're jabbing in Rhythm, they can only roll away or time a shield grab. You can parry the grab, and read the roll. If they roll into you, they get jabbed. If they jump OoS, they get Jabbed. If they spot dodge OoS, they get jabbed. If they keep their shield, they'll get Jabbed or their shield will break. Out of this pressure, you can read one of the safe options and get a grab, or grab parry, or throw in a double jab to stuff a shield grab. and you can get DJC N-airs and grabs and whatever you want out of a landed jab.
As far as D-air goes, I get them onto a platform above me and then jump up with a D-air. If they shield, I get as many hits in as I can, (Because D-air eats like 70-80% of their shield) and then I do what most of you haven't considered, not land (of course you L cancel) and parry and stuff, but Ledge-cancel. If you ledge-cancel, you don't have to L cancel, and you'll be out of range for them to retaliate. If your opponent is unfamiliar with that, they'll try (and fail) to shield grab you. Then you Rising F-air them to a N-air or U-air. Ledge-canceling is my thing, it makes combos even more ridiculous and fast.
Also, if you L cancel D-air, you can double jab immediately or do the timed jab.
Every Yoshi should have ledge canceling perfected, it's an amazing tool for Yoshi (and for any character for that matter) and it'll speed up your combos and set-ups will appear that you didn't think about.
I hope this helped guys!