Assess the existing situation by closely examining the conditions before you begin. Once you get inside that wall, you're stuck dealing with whatever you uncover. Then you are forced to be reactive. Make sure contractors have the correct materials on-hand and order spares of special orders. It might cost a little more, but if a plumber is walking up the stairs with a special-order toilet and drops it, the entire project is off schedule.
Guidelines are clear for new construction.
However, renovations can present unique challenges depending on the age of the building and the extent of the project. Consider all the trades. Specify high-quality fixtures and products. These rooms take a lot of abuse. Whenever possible, build in a contingency plan on everything, including labor and materials.