I build myself a Mendel 3D printer a while ago (I wrote a blog about it) and could not find an easy solution to print out my stuff. I stuffed away the project for a while and just recently I pulled it out again.
I found a software called Repetier that works as a charm. Printing objects from SketchUp is now really easy, no "extra" conversion steps in between.
You need to install an STL Exporter to SketchUp. I found this one from Flight Of Ideas that says SVG exporter, but it can export STL as well. After the plugin has been installed you get a new tool "Export SVG or STL". It will export all objects that are selected or everything if nothing is selected.
If you prefer another STL export plugin, feel free to try them out until you find one that works well with you.
From Repetier, just open the STL file and hit the slice button. Repetier does the slicing for you and supports a few 3D printers, CNC milling and drill.
You may fiddle a bit with the slice configurations, but it took me only a few hours to get things darn good.