umm.. I usually do really well against marths and IMO.. yoshi's story is not so bad. The thing with most marths is that they have the same predictable patterns (most of which dark.pch already explained) the trick is to abuse peach's priority while keeping your distance.
obviously turnip camping is best against a marth who waits for you to approach. but when a marth dashes in and is most likely going to grab, the slaps (A + A) is a great way to keep him back. they will most likely go into sheilding as a reflex. use this time to grab them. down throw at low percentages, forward throw at 120+ percentages (earlier if you can get them far away from the ledge, use your judgement)
for the down throw. if a marth DIs forward, tech chase + Dsmash is great for damage. for a marth that doesn't not DI, Usmash -or- chain grab works pretty well.
when marth approaches from the air, UP + B out sheild helps you to keep your spacing, and if you land w/o lag (dropping the parasol) you may have time to jump and Nair them.
Peach's pressure game can be really overwhelming to marths. learn to use it to your advantage w/o being careless, and you should be fine.
IMO..
FD, Dreamland, Kongo Jungle, Jungle Japes, and brinstar are best for peach against marth. the first four because of the high ceilings. peach will live quite a long time. marths are most dangerous at peach's lower percentages. after about.. maybe 80 - 90, they cannot juggle her.
yoshi's story, rainbow cruise, mute city- they are OK counterpicks.. but with rainbow cruise and mute city, you have to be careful because there are times when neither player can sweet spot during a recovery and it can be harmful it you are struggling to recover.
I don't have many recent vidz, but if you check ww.youtube.com/handofthephoenix, u'll find some peach v. marth vidz ^_^