There are a few ways you can reduce file size. The best way to reduce file size it to model smartly from the beginning. Assuming you do not want to remodel anything, there are two simple ways to clean things up a bit.
The first way is to use the purge unused option in the Statistics section in the model info window. This should get rid of any unused assets that may have made their way in to the file while you were creating it. The next thing I would recommend would be the cleanup³ extension by ThomThom (https://extensions.sketchup.com/en/content/cleanup³). This extension is a more aggressive way to clean up a model so be sure to back up your file before you use it.
If you are willing to do a bit more modeling, you can reduce file size by using components for objects that repeat in your model. You can save even more space by reducing the number of sides that you generate with the circle tool when you create a circle. This is especially helpful if you plan on using a Push/Pull or Follow Me tool, as it will reduce the number of faces created later on. Finally you can save on space by only importing what you need from the 3D Warehouse. There are many highly detailed quality models on the warehouse, sometimes you do not need all the details that a component comes with. Do not be afraid to strip out excess detail you do not need.
If you are willing to upload a few screenshots of your model or better yet, share it via a dropbox so I can look at it in a more detailed manner, I might be able to make a few more suggestions.