Crowd management becomes a big part of DDR once you start to get good ;D
IMHO it's the coolest part of the game.
Crowds are attracted to a few things:
1) something they know they can't do themselves
2) something they don't understand
3) something that you don't make look easy
4) other crowds
To expand on this, you cannot do a song that your average non-DDR player will believe that they can do themselves. Playing a light song and AAAing it will not get you crowds, as technical as that is, it isn't flashy.
Playing a song that you can do in your sleep also won't get you very big crowds. If you can play B4U on heavy and you snooze through it, barely moving your feet, with your hands in your pockets, you won't get big crowds.
You also won't get crowds by flailing madly on a song that you aren't good at. If you're no good at gallops, trying to do Saints Go Marching on heavy will not gain you big crowds because you won't look "good". The only crowds you'll get will be random people stopping and looking at you with puzzled expressions, then moving on.
To get crowds, you must play songs which are somewhat challenging for you, but something that you are still fairly good at. Once you start attracting crowds, it'll grow like a magnet, because people will come just to see what's going on.
Flashy mods are good to maintain a crowd. Technical stuff like shuffle will not get you crowds, neither will reverse because the crowds don't realize that these mods actually change the arrows. Flashy stuff like hidden, sudden, and occasionally boost will maintain your crowds because they will be impressed by the fact that the arrows are different and unintelligible.
Also remember that crowds often look at your feet, and not the screen. If you start messing up on a song, don't just stop and stare blankly at the screen until you get it back. DO something, even if you're hitting the wrong arrows it'll still be crowd-worthy. I've failed MaxX Unlimited and gotten applause for it just because I hit random arrows when I didn't know what to do.
Other tips are to be FAST in your song selection. A big crowd can dissipate inbetween songs if you take more then 15 seconds to pick your next one. Don't spend long periods of time in your score screen either, nobody understands it, so they don't find it interesting.
Also, the final thing is to always one-up yourself. If you played a 6 foot heavy song and got crowds, don't play a standard song. They'll go away. Crowds base their opinion of you based on what they just saw you do, so if you are looking for crowds, play a 6 footer, then a 9 footer, then try a 10-footer, or whatever is within your skill level. Don't play the 10-footer first and then jump to the 6 footer, because the 6 footer will pale in comparison to the 10 you just did. Work you way up to the harder, more impressive stuff.
Turnarounds are also very impressive to crowds. They love it when you don't look at the screen, even for a few seconds. I don't recommend knee-drops and hand plants because it's hard to make them look good. Crowds sometimes find it entertaining, but usually not impressive.
What else... hmm....
I personally play any 8 or 9 foot song when I want to impress crowds. 10 footers work as an encore song.
I NEVER play oni for a crowd unless it's like demon road. If I do I make sure you have a friend acting as a spotter in the crowd leaking comments like "he's only allowed to miss 3 arrows total, if he misses more he fails". People get impressed with pressure like that, and usually they'll make the connection between that comment and the little battery on the corner of the screen all by themselves.
Nonstop works quite well though simply because they are usually harder heavy songs, you don't have as much pressure to hit all arrows, and the songs are back-to-back. Often nonstop courses that are not random-song courses will increase in difficulty as they progress too, which was one of my previous tips.
Yep, so... those are all my tips on crowd management.