What's a good method for shield pressuring? I often try spacing aerials and d tilts and stuff, and grabbing in between if I see an opening, but it often doesn't work at all (especially since that stuff can be crouch canceled at low percents). Basically, I'm either doing the right thing very badly or not doing it right at all. I still need a lot of work on my spacing (of course), but do you have any tips anyway?
You have to know what you're trying to achieve by pressuring a shield with Marth. You can't effectively use it to actually wear the shield downs significantly as with Falco. What you're trying to do is force the opponent into making a bad decision and then punishing him. If you're good enough at this you can make your opponent feel trapped in his shield, which again pressures him to commit to something that's easily punished because he's nervous.
Also, since you're not a spacie, you NEED TO TIP EVERYTHING. Not only does this put you out of range of a shieldgrab (even in dittos), the tip of Marth's sword also causes more shieldstun and probably more shield damage, though I'm not all too sure about the latter.
Dtilt is really safe. IASA frames are absolutely amazing, so if you just want to wear their shield down a little and be safe, just dtilt. A VERY simple trick is just dtilting repeatedly using IASA frames. Either you shieldstab and you can tech chase, or they roll/spotdodge and you can grab them. Shieldgrab doesn't work if you spaced it right. The only really safe options they have are wavedashing BACKWARDS OOS (I'm pretty sure you can grab them before they can do **** to you if they go towards you) or jumping away. If they WD away, just stay close, camp in their face, and pressure them. If they jump, uair and utilt **** them.
A lot of the time, doing nothing and reacting works wonders. Essentially, you have the same options as out of dtilt except you're not actually doing a dtilt. Watch out for aerials out of shield against characters with good range like vs. another Marth, so you might want to crouch. You still have the same options plus the benefits of broken CC grabs.
Being proficient at pivots helps a lot. If you're close to their shield and want to move into a safer position, you can dash away and empty pivot to space yourself correctly, or if you want to be fancy and slightly more aggressive you can pivot an ftilt on their shield. Again, correct spacing is crucial. You can also just run
through their shield and pivot grab, or if you're standing at the normal shield pressure range, run through and FLT grab. Also, running through people and just doing an empty FLT pivot (so basically FLT grab but w/o the grab) is amazing because they can't do **** to you. Lots of people jump to avoid FLT grabs, and if you don't actually grab, you can chase them with uairs and **** them.
If you have frame perfect timing, I think you can auto cancel nair and dash dance away and grab, though I wouldn't recommend that unless you're super good at that and believe they won't be able to shieldgrab you. Another simple way of getting behind their shield is to shffl an uair behind them so that it tips and then spacing yourself right simply by walking or turning around. Cactuar does this quite a lot in dittos.
I probably forgot some stuff, but there's some possibilites you have.