Vests.

 Wear a vest.

1. Vests look smart and slightly old-fashioned. Like stockings or tweed.
2. Perfect for light layering weather: warm torso, cool arms—when a shirt is not quite enough but long sleeves are too sweaty or constricting.
3. Makes a nice change from a cardigan.
4. A vest extends the life of an old shirt, as long as the collar* is still nice.
5. Also conceals a multitude of shirt sins: un-ironed, stained, thin, lost button.

Also applies to waistcoats. Patrick Jane The Mentalist wears waistcoats. I think waistcoats are a bit too fancy for everyday wear myself, but I do think they should be more widely worn.

*Vest over a t-shirt is also a look I'm quite fond of at the moment. More soft and comfy than a collared shirt, and it's more urban than old-fashioned. I'd like to see it being worn. I'm trying to start the trend.