Yeah I have lots of tips- just got in from a ride...so might be a bit scattered (got a flat tire from a 930" thorn then did foot trials for 2 hours).
Ride natural, all the time. The right kind of natural...I'm not talking about a rock garden with flat ground- big connecting rocks, be it grass or hard rock...ride things that will give out underneath you- do everything from/into sand, or pebbles, or very small rocks. As long as they move around you're stabilizing muscles will have to work 10 times harder. Ride this type of stuff, no matter how frustrating, for a week- then go back to riding on some concrete- you should have a lot more control over your bike. This should give you enough incentive to try and ride that kind of stuff regularly.
Monkey see monkey do....don't watch videos of jagged riding styles, watch over and over people you feel are very smooth and you want to emulate. I've always watched vids, over and over...since you can't see them in real life, you just have to pound it into your head. Hopefully you will take something out of it.
Lift weights. Nothing crazy, there are lots of stupid beginner/sport trials riders on certain forums who are total meat heads and think you have to power lift to get good strength. Don't waste your time...technique will always be #1, concentrate on that, but if you have one or two days a week to work on all body lifting try to do it. The added strength will really show off in your bike control. Thats what being smooth is, having complete control over every part of your bike.
Your bike set up and frame will affect how smooth you are as well. The extreme of this is a mod vs stock, everyone is smoother looking on a stock....when it comes to stock frames, I was smoother than kleivard having sex with a lubed up sperm whale on my crescent. It was a 3.4 pound really flexy frame. When I got my koxx it was a 5+ pound stiff beast, I was never very smooth on it comparatively speaking. Right now the woodman is very similar to the crescent, but some of my moves that I have now are very power driven and its difficult to be completely smooth. I personally prefer a softer frame than a stiff one.
Finally, some of it is just natural. I don't know if its mindset, or body type, but some people just cannot be smooth no matter what they try. Others just are. I've seen tons of people who ride urban all their lives be just horribly shady and gooney looking, but I've seen others who have the same background look sexy as fuck. My first year of trials was spent on a loose dirt packed hill will logs on it, I never bothered with rear wheel moves or anything like that till I got to college. Most kids these days just start out trying to drop some 5 foot loading dock, when they should be learning how to roll through a hard set of rocks.
Sorry if I'm rambling :dunno: Time for shower..