Dynamo lights FTW.

Non-rim brakes. I probably would go for hub brakes but they would be less versatile (I’d be more worried about overheating on long descents if I used the commuter for touring) and, more to the point, are far harder to find in this country.
A fixed gear will do the same job, but I kept breaking things (cranks, BB axles, a stem and a frame) on mine to the degree that the KISS principle broke down

Mudguards (all year round, take the road bike if it’s nice weather and I don’t have kit to take in)

Rack (I always wonder why more people don’t run racks/panniers – so much nicer having the weight on the bike than on you)

Warm clothes and a bike with ice spikers fitted ready for the dodgy ice days in winter.

Rack (I always wonder why more people don’t run racks/panniers – so much nicer having the weight on the bike than on you)

There are a couple of downsides:

1) My load wobbles around on my rack no matter what I do – probably down to the shape of my frame and the way the rack has to be fitted.

2) Faff. I’m going by bike and train a lot, and it’s far easier if everthing’s on my back because I can just chuck my bike on the train rather than having to put luggage off and on. Also when you get to work it can be more of a fiddle.

I still do use panniers, mainly because I’m carrying too much for my back, but also the sweat factor – if it helps me avoid a shower at my destination then it helps.

I use a saddlebag – specifically one of these http://www.wiggle.co.uk/ortlieb-large-saddle-bag/ Takes enough if you only need shirt, underpants and socks each day along with tools and a waterproof. Can attach/remove in a few seconds but is small enough that you don’t have to remove it to put on a rack on the train.

Does depend on being able to leave a lot of stuff at work though.

I’d really love one of these for weekly travelling: