Over-dancing is something like that awkward stage of adolescence, a development stage that most intermediate dance students go through. We know a lot of what to do, but that lack of experience leaves a certain immaturity to how we do it.
Some dancers never quite move out of this stage, instead treating it as a mark of skill to show off everything they can do, each and every time they are on stage. There are lots of factors that have encouraged this in the dance world, but it is partially to do with the change in audiences. Native audiences and dancers are usually looking for a feeling: to experience tarab, or to feel connected to their culture or their community. The role of the professional bellydancer as hostess/party starter is one part of why we want to avoid over dancing. But it also allows us to relax and enjoy our own dancing, and makes the performance that much more captivating for the audience.
It is part of Arabic artistic styling to take a concept and really explore it. Think of Om Kalthoom, she would pick a phrase and improvise different ways of singing it, driving the audience crazy (in a good way) in the process. No less than Raqia Hassan has said that a choreography only needs 4 moves. If you use your movement variations to explore all the ways you can approach or execute it this is not as repetitive as it sounds. Think of your movement vocabulary as having a few root movements with lots of ways to modify each. For example, instead of an omi, a flat hip circle, and a big tilting Dina style hip circle being 3 different moves, think of them as all being variations of a hip circle; the same way "will dance", "danced", and "dancing" are all related to the root verb "to dance".
Bahia, of Austin Bellydance, explains a lovely way of thinking about this in her Combinography DVD: She explains that "your dance is a poem, not as an encyclopedia." Check out this video of Esmeralda performing a drum solo, her stillness creates contrast and makes the moves she does perform that much more dramatic.
To continue the language metaphor, if you're going to talk fast you have to enunciate really well, but you don't have to talk fast in order to say something profound. When you over-dance, it can be because you have a lot that you want to say and it all comes out at once, or because you're unsure of your expression and worried the people you're talking to (audience you're dancing for) aren't following your train of thought. The irony is, the more you add in, the harder it is for them to follow. After you have been dancing while, maybe performing full length sets, maybe just having been in a bunch of showcases, you learn that you can do just one thing at a time and start to feel like you have more time to say it. And because you have already said a lot of things in previous performances or practices, you can focus on going more deeply into one topic or theme (or style, or movement).
One way to overcome over-dancing is to just dance A LOT. Improvise and choreograph until you feel like "been there, done that", so that when you go to do a new piece you don't feel the need to include everything you've learned. You want to reach a point where you are making each piece unique by picking just a few of the things you've learned, almost rationing those things, and one way to get there is to feel like you've used up and said all those things already. This approach basically amounts to getting the over-dancing out of your system. How long this will take depends on how energetic a dancer you are to begin with, and how often you have the chance to perform. Another way, or a way to help you feel comfortable with doing less sooner in your dance journey is to use music mapping. It lets your plan out your piece and ration out your artistic choices ahead of time before the panic of performing, and the distorted sense of time that comes with it, tricks you into thinking you haven't done enough to hold the audience.
I recommend, especially if Egyptian style is your thing, to start with practicing beledi progressions. Not only is the earthiness of it helpful for your technique, but the music's structure comes pre-mapped. Ranya Renee has an excellent DVD set about it, and you'll notice how the progression gradually builds over several distinct sections, each with certain steps or energies traditionally associated with them. It is wonderful fun to dance, and it teaches you to pace yourself.
Alternately, you could work with the 5 to 7 part routine structure, and practice saving different categories of movement for each section of the set. If you have a hard time pacing yourself over a 5-10 minute beledi progression, having to save enough energy to finish a 20-45 minute set with a bang will force you to ration your energy!
Also, with learning to not over dance, having an understanding of musicality and phrasing is really helpful to give yourself the confidence to wait before you switch to doing a new thing. It helps you avoid the trap of feeling like you need to change sooner than you really do, because the music will keep you from rushing when you're onstage and your sense of time changes. You can use the phrases of the music as your cue to change moves or directions etc, rather than giving in to your nerves and leaving your dance seeming disconnected from the music. I wrote more about musicality in another post but basically: use it to give your improvisation structure. If you've picked an interesting song, then you can let the composer to the work for you! If the song is interesting, and your dance reflects the music (as raks sharki/dans oryantal always should) then your dance will be interesting! No need for nerves pushing you to change up your moves when you think the audience is about to check their phones, just hang in their and wait for the phrase or verse to change, then relax and enjoy!