This should be obvious, but it can be easy to forget that the central theme of this game is physics. You are trying to get these little Gooballs to make the buildings you need. They are limited by the games physics though. If your tower is swaying badly, then you need to fix that before you go any further. Remember to keep the weight fairly balanced on both sides. Once it starts swaying, it is really hard to stop it without using a time bug.
You should also learn to balance your goals well. Most levels challenge you to make a strong structure with the least Goo possible. This is challenging, since the most effective way to do this is usually to make a narrow tower. Narrow towers are the least stable though. Figure out a good blend between the two strategies. A good setup is usually to have about 5 or 6 Gooballs forming a strong square base.