(12 weeks PP)
There is no way to actually physically manipulate to tighten your skin. Your skin, depending on its elasticity, actually retracts on its own. If you have more elasticity, your skin will tighten up quicker. If you have less, than it can take some time; years, maybe even months to get back to where you were. If your skin stretched extremely beyond its means, then surgery may be the best option for you.
However, there are some great ways to build the muscle underneath the skin to give a more toned appearance.