How can I effectively use Google Sketchup to render walls, doors, and windows?
You need to learn to use Components in SketchUp; they do exactly what you want. They do not "stick" to other parts of your model and won't affect other lines/etc but you can still snap to them when drawing.

Also, you can go into a component and edit it, and the changes will be reflected in all the copies you have made of that component (or, you can break a single component free and edit only that one).

Additionally, the paid version lets you create Dynamic Components; e.g., a cabinet where you can alter the reveal around the door just by entering a number, or a stair where you can set the run, rise, and number of treads without drawing anything new.

Components will do it for you!