The most important step is to make sure your model is a SketchUp “solid” (not really a solid in the sense used by other modeling tools - a group or component that is a closed, airtight surface with no extraneous faces or edges). ThomThom’s SolidInspector2 and TIG’s SolidSolver are excellent extensions for checking and correcting issues with solids. Depending on what you are trying to print, you most likely also have to build the model at an exaggerated size so that small edges are not lost, and then scale it down to the real size just before export as STL.
Also, I want to note that we now have a Printables feature on 3DWarehouse! Printables is a feature that makes it easier for millions of 3D Warehouse users to create, share, and print STL files. It’s built directly into 3D Warehouse, so there’s no need to download any additional plugins. Simply upload your SketchUp model to the 3D Warehouse, make it public, checkmark it Printable. From there, 3D Warehouse will hand your model off to Materialise’s Cloud Services which will analyze your model and automatically fix some of the most common issues that are known to hamper 3D Printing workflows. They will then generate a watertight STL file and send it back to 3D Warehouse where it will be available for you to download.