1. MAKE FRIENDS WITH THE MIRROR- Practice body rolls against a wall or mirror, focusing on 'peeling yourself off of the wall'. For a downward roll, start with your head and follow with your chest, stomach, hips, and knees. For an upward roll, come back up with your knees, hips, stomach, chest and finally head. This will give you the muscle memory, and muscles in general, to do great body rolls while dancing.
2. WATCH OTHER DANCERS- whether on the dance floor or the internet, observing can be your greatest learning tool. Don't allow yourself to be limited to the styles present in your area. Watch dancers from around the world to see what styles are out there. No matter where you are observing, pick dancers who you admire or who have something you'd like to incorporate into your dancing. Make it your goal to learn something from someone each time you go out (congresses are perfect for this), and you will add lots of variety to your repertoire.
3. DO SIT-UPS- strong abs give you better balance, which allows you to to be more smooth and centered on the dance floor. This is most important for spinning- the best way to spin is with a nice, tight core that allows you to spot and stay balanced. Try doing ab-strengthening exercises just before going to dance class or out to the club, and it will likely make you feel much more controlled. That is the short term fix- as for long term, strong abs will help you become a better spinner in general since you'll have more strength to whip around faster, and will also allow better strength for attempting more advanced moves.
4. RECORD YOURSELF- The video camera is your friend! Really, it is. This is perhaps the best tool out there, as humbling as the experience may be. Have someone video tape you social dancing, in practice, in the studio with a partner, wherever you want. Later, watch the video and see what you think. Sometimes we are our harshest critics, and for this very reason, it will allow you improve at a much faster rate. We don't always have great perception of what our bodies are doing- we make think that we are doing something correctly, but then on film we'll see that we are not, and this allows a better connection between mind and body. This is also a great tool for dance teams- record your routine in practice, tech rehearsal, and performance and analyze it. That way you know if you are in unison, and if all the movements are being done the same way.
5. BODY ISOLATIONS- In front of a mirror, move each part of your body in isolation. Be careful to ISOLATE the movement, only move one part at a time, no matter how slow you go. This works great to slow cha cha music, so each movement can take a couple 8 counts. If you dedicate some time each day to practicing these isolations, your body movement will improve and will become much more natural out on the dance floor.
Shoulders: Start with one shoulder, rotating in backward circles, then forward. Then the next shoulder, both directions. Then move both shoulders at the same time, starting at a slow shimmy and working your way faster and faster.
Hips: Move your hips to each side so that you feel a good stretch around the hip bone area, to one side first, then the other. Make sure your knees are bent so that you increase your range of motion (and don't get hurt). After you have gone to each side, go forward and back. Then try the figure 8- have your hips follow a lateral figure 8, or I suppose infinity sign. This one is trickier, but VERY IMPORTANT for improving your basic step and other footwork. I like to imgine each hip bone drawing part of the figure 8, the left starts the motion, making a clockwise circle, and the right hip continues the motion with a counter-clockwise circle. After the figure 8 drill, move on to large circles in both directions, and you are done.
Core (rib cage): Standing with arms out to the side, shift your rib cage from side to side, being careful not to move other parts of your body (like the hips). Once you have the side to side, go front and back. Then go front-left-back-right-front and gradually speed it up until it becomes a nice roll, and then repeat the same for the other direction. For the front to back motion, you can also practice popping the chest out to get that sharper look.
Hopefully these will prove to be useful tips for the dancers out there who want to take it to the next level. There is no substitute for social dancing and classes, but doing these things as a supplement will definitely speed up the process and make you a more complete dancer. Have fun!