You shouldn't have to routinely spam the follow key when you're just moving around. You will have to if you whack something in melee, perform a channeled spell or do one of a small number of other things though.

If you're losing follow just moving around then I would guess that it's your machine that's the limiting factor and you'll probably want to turn the display settings down a tad or lock your framerates if you haven't already.

So... under what conditions do you see your follower become a left-behind-er?