During this transitional period, the weather can do crazy things. In the mornings, it's in the 50s. By 11 a.m. it's in the 80s. So you need something that will keep you warm, while still being lightweight so as you don't melt when the temperature rises. Enter my next essential item:
the denim jacket.
Denim Jacket~Levis, Camp Shirt~Gap, Chinos~Dockers, Belt~Army/Navy Surplus, Desert Boots~Aldo, Watch~Timex, Sunglasses~H&M
Treat them like you would a pair of jeans and wear them with everything... except a pair of jeans. It can be done, but it can be tricky. One has to be light and the other has to be dark. Break this rule and you'll be rocking what's known as the Canadian Tuxedo: not a good look. And if you get one slim enough, you can use it as a layering piece and wear it under your blazer or wool coat. Also, if you're up for the challenge, you can buy one in raw denim and break it in yourself.
Lastly, the awesome thing about denim jackets is that you can save a ton of money and go vintage/thrift and get one used. There are a plethora of 70s & 80s era jackets that seem to just pile up in secondhand stores. Just be sure to take it to the tailor to get the fit right. It should have high armholes, be slim through the sleeves and body, and stop right around the waist. Otherwise, you'll just be walking around in a denim blanket, and random strangers will hand you quarters.