At the very very beginning solo tech practice should probably just be trying to consistently shffl and wavedash. Think about dash dancing and it's uses along with doing exercises to control it, like dash dancing across stages or dash dancing on platforms of various sizes to control length.
Once you can consistently do those work on general movement, so what I like to do is just wavedash and dash dance around (wavedash to cancel your run animation so you can dash dance) and try to move around or to different spots on a stage. I think a pretty okay way to practice dash dance/movement control is to put a level 9 peach on FD or something. basically you want your dash dance to be far away enough so that she can't hit you with anything (dash attack especially) and close enough to be able to punish every time she pulls a turnip. or you can play against a sheik if you have a friend and tell him/her to just dash attack a lot and try to space around it and punish. simple things.
beyond that you don't need much; marth get's a lot from pure fundamentals. After you're consistent with those I would definitely look into practicing:
pivots (
http://www.youtube.com/watch?v=WuCCmymFpMU)
instant up airs (really useful for juggling at combos; basically have to get the up air out as soon as you jump, then normal fast fall and l cancel things)
chain grabs (these are actually really essential for the spacies match-ups; it always helps to develop your punish game)
ledge shenanigans, like waveland from edge. If done frame perfect you get 4 frames of invincibility I think.
platform shenanigans (?) marth doesnt get much from these in my humble opinion but they look cool and you can set up kills I guess? things like wavelanding onto/off of platforms
other than that just play lots of people.
