I suggest practicing your shffls ON the computer, as in hit him with them, otherwise you'll learn the wrong timing for the shffl (empty shffls are differently timed then shffls that hit something).
As for "cool lil combos", the most popular and well known Marth combo is just simply repeated SH double Fairs. You could also try to fit a Fair and a Uair in one SH, which is something I saw Kizzu, an excellent Brazilian (I believe) Marth player and one of my idols, doing in his videos. The Uair puts your foe high enough to (if you l-cancel the Uair. Learn your shffl first
) go into a Bair -> Fair -> spike if you're near the edge. Theres a match against his brother Walk (who is also a Marth player, but was playing Shiek at the time) where he uses his SH fair+uair to set up a long combo, bringing Shiek from 16% to 117%, then finishes with a tipped fsmash for the KO. Beautiful
As for upB and B, his upB can KO if you hit with the opening animation (you can KO with a reverse upB on the ground. In the air is possible, but takes some pretty crazy timing, and a spike is usualy a better KO option in those situations anyway). It's like Luigi's upB in a way, in that the sweetspot is in the first few frames of the hitbox.
You can edgeguard with NeutralB (I know that recently, Ken has liked doing this more then usual), but as mentioned already, between your spike, your fsmash and your dtilt, you won't find yourself needing your B too often. You could use it to finish a combo thats offstage, where a spike would result in a SD, because it sends furthur then a fair.
PS: Marth can't DJC... >_<