Using sketchup to plan page layouts! I had known about it while working on the first chapter of my comic, but I kept putting off using it because booo 3D scary D:
BUT now I've been using it to plot out complex environments in my comic, either using downloaded models (You can find a model for almost anything on their website!) or building my own from scratch. It's been a HUGE time-saver, and ensures consistency from panel to panel so I don't have to remember where all the furniture is located in a room when I switch angles. I make a lot of changes to the drawing on top of the sketchup screenshot, but it acts as a great base to build upon and saves SO much time I would've spent plotting perspective lines (though you should still learn perspective D;).
You can also download things like vehicles or people models for crowd shots. My latest comic pages had a motorcycle in them and I guarantee I would've spent at least 2 extra hours per page if I had to draw that thing from scratch every time.