What's the best way to avoid ngons and tris in a model? Basically the question says it all. I'm asking this because I keep getting a lot of ngons and tris with the model I'm working on. And I plan on letting people use this model for whatever they want(whether it be mods or animations) and I don't want it to be filled with ngons and tris.
What has worked best for me so far is to (A) start with quad-based meshes, such as cubes, UV spheres, and cylinders and then (B) add new geometry by using loop cuts (Ctrl+R) and extruding faces (E) in edit mode.

You can then move vertices manually in edit mode or use sculpt mode (with Dyntopo off) to shape your model without adding vertices or creating ngons.