Good lighting to see and be seen-with plus spare batteries and back-ups. Site lligts at different positions at the rae, have one solid and one flashing.
Reflectives, ankle and wrist bands are cheap and available (e.g. - slap-wraps at
Halfords) ankle bands are particularly effective at night.
Peaked cap is very useful like a cycle cap (Prendas £6 ish) which can be worn under a helmet if desired is good to keep the glare from oncoming lights out of your eyes.
Flexible clothing's good, temperatures can change rapidly at night. Buffs are useful too.
Cold feet are a killer, overshoes are brill, but sandwich bags work as a cheap and lightweight alternative (over your socks).
Hot drink? Take a flask.
Remember to drink and eat also, I find at night I tend to do this less than during the day, don't quite know why.
Night riding's ace.
Edit: A good head torch such as a Petzl Tikka plus is a great addition, allows you to scan the hedgerows and gives a high level point of light to oncoming road users. One of my best purchases.