Lasting thing: practicing wave-dashing.
If you are having a tough time (so am I in UMvC3), I recommend just practicing this exercise: Dash, down, Dash, down. That's it. Don't worry about being able to wave-dash infinitely yet, just focus on being able to cancel a dash into another one quickly. For some characters just being able to do a "double dash" should be enough to get very close to the opponent, or even chase them down after a air splat mid-screen so you can get your OTG.
Also try mapping your dash to L1/L2/R1/R2, which ever you like the most. Trust me, it helps a lot. So lets say you have it mapped to L1, then you need to be practicing: L1, down, L1, down. I mentioned "double dash" above, and that's what I like calling it because it helps me with the next part.
So when you want to do like 4 dashes or whatever, in your brain you need to think of it like linking a "Double Dash" into another "Double Dash". DONT think about it as doing L1, down, L1, down, L1, down, L1, down, over and over again. Just think "Okay, I have to do my double dash that I've been practicing, and then do another double dash quickly after"
Then of course if you need to do 6 dashes for whatever reason, just think of it as doing 3 double dashes or whatever. Eventually (hopefully lol) you will get to the point where you are so comfortable in your double dashes into double dashes, and you are so used to the inputs, that you will be able to do it infinitely.