Difficult.
What I do (not to say I'm un-mumsy and über-stylish! But I don't see my wardrobe in the "do not" list...)
Mostly no prints, unless it's a scarf and even then I don't like them.
Lots of neutral colours. I suit wintery colours so I wear lots of black, silver, pewter, etc. With some teal if I'm feeling adventurous. But all cool colours that I know suit me. I think mumsy sometimes comes from washed out warm colours on people that don't suit them.
Figure emphasising. The good bits! I'm very fussy about clothes silhouette and not looking blobby.
Plain good quality material (no embellishments). If fabric looks cheap (doesn't mean it is cheap!) I don't wear it. I don't spend loads on clothes but I like them to look expensive even if they're not. I also shop sales almost exclusively for the same reason.
Jackets. I love jackets, they instantly dress things up.
Overall look. I picked one, put together a load of pics on pinterest and refer back to my "look book" when I'm shopping. In an attempt to get it all to go together! (Not always successful). I also try to purge regularly for the same reason.
Not a failsafe method but that's my approach. And I wear comfortable clothes so haven't sacrificed that.