This is where I put useful tips and tricks I have came across.
If there is a problem with your model's joints, make the model transparent and then select show joints. This will show where the joints are and help you adjust the xacro to make the joints as they should be.
If you use STL model files, make sure they are centered and rotated correctly before making a URDF or Xacro files. This makes it easier to place the parts by: reducing the efforts of rotating the parts using the URDF code and making the parts placement symmetrical. I used simplify3d to center the STL parts. I wish I had done this from the get-go.
killall gzserver - a good thing to try if your Gazebo is crashing or not showing some of the model parts.
catkin_make clean - Cleans up your workspace.
5/18/17
Install Ubuntu on an external drive, and then install the version of ROS that you know or that the guide you are using calls for.