I know exactly what you mean. I like thyme, but it gets so woody in the middle that it looks really awful as it gets older (which can be said for me, too, I suppose). The best thing to do is shear back your ground covers after they bloom. Some will perk up as they grow back. Another trick is to plant different kinds of groundcovers together, and also plant some taller plants around them that will cover up their less-than-loveliness as their season passes.