There seems to be a consensus on here that fit and silhouette have an important role to play in frumpiness, or lack there of. For what it's worth, the things below are what I believe you have to be veeeeeeeeeery careful of as they can be frump tastic if not deployed with care:
- midi skirts. Especially when pleated and worn with flats. Alexa Chung only gets aways with it because she's a super skinny fashionista.
- beige. I.e. the shade of beige that the walls of most municipal buildings are painted in. It's hard to look stylish when you're the same colour as your wrokplace.
- baggy cardigans that end mid-thigh. Shapeless, looks like an item being worn for warmth only.
- shoes with a low (1"-1 1/2") block heel. The Guardian's fashion section says these are on trend for 2013. I, for one will not be partaking as they just scream, "sensible!"
- shoes that look orthopaedic, but aren't actually orthopaedic shoes. It is possible to buy comfortable shoes that look good. It may take a bit more work, but the results are definitely worth it.
- tapered polyester trousers in a plain colour. Just joyless and make your bum look bigger.
- pastels can be tricky on an older woman. The wrong shade can make you look washed out.