I find that two things help me achieve that without turning on my blender (or having to clean it when I'm done).  First, start with a perfectly ripe avocado.  It will mash up easier than one that's not quite ready.  Second, keep mashing with your fork until all the avocado chunks are broken up, then switch to stirring with your fork.  You'll get a smooth avocado puree every time. Alternatively you can also use a mortar and pestle like this one and let your big kids help out!