Decent footwear - Probably the most important essential here. The likelihood is that you will be walking a lot, so you'll need substantial waterproof footwear if you are going somewhere cold and rainy. On the other hand, if you're likely to have boiling weather, don't take the hottest shoes you own because that's not going to be good for anyone!
Travel guide - As you are only there for a short period of time, the best way to learn about a city and get lots done is to use a travel guide. Marco Polo and Lonely Planet are my favourites, and they include a lot of information about the history of the place you are going to visiting. I love to research the places I am going before I go so I get the most out of it!
Sunglasses - Even in the Winter! One of the things that is a must, is to bring sunglasses. Even the winter brings a glow, and obviously you'll need them in the summer. I think you are much more likely to enjoy yourself if you can see and aren't squinting 24/7! Plus, sunnies also look pretty fantastic in any outfit shots you might be considering...
Appropriate Luggage - Packing as lightly as possible is obviously a given, but bringing appropriate luggage is probably even more important! If you know that you have an early flight but can't check in until the afternoon you are going to want something you can carry easily without doing any damage to your back. I have a great lightweight suitcase that I take - it's spacious but also fits airline requirements...well worth investing in.
An outfit you feel comfortable and amazing in - There is nothing worse than going somewhere and feeling uncomfortable and self conscious. This is why choosing your outfits wisely is crucial. I tend to pick things that I could wear both in the day and the night-time to save on packing. Feeling comfortable and confident in what you are wearing means you'll forget about yourself and focus on your surroundings!
What are your city break essentials?