I love low-light photography!!! There is endless potential for creative solutions at night.
To begin, everyone here is right so far. Fast lens, tripod, off-camera flash, long exposures etc. Let's talk about how to get YOUR picture though. You want your subject lit and well defined with the cityscape as a well-exposed, complimentary light source.
Equipment: Camera, lens, off-camera flash, tripod, magenta filter, green gel.
The predominant color coming from the city is green: fluorescent, sodium vapors, etc. Slap your magenta filter (or custom WB or fluorescent WB) on to clean up your background. Gel your flash green so that it's output is the same as the city. Now get your exposures down. If you're using a nice CLS like Nikon's, than this is pretty easy. If not, get out the old ruler and start working your distance formulas. I like to underexpose background be 2/3 and subject by 1/3 to maintain moodiness of scene and separation of subject to background. Get your EX set and let'er rip!
Of note: Subject blur will be minimized by the strobe. Depending on how long your shutter runs you may get some movement anyway. This can be a good thing. Work your equivalents until you achieve your desired effect. Also, the addition of magenta really sets the sunsets (if you have any) off! They look great! Second, though fast lenses give you more light and thus speed up your exposure times, they also crush your DOF. This is awesome, unless you want your cityscape recognizable. I LOVE my f/1.4, but I would never go that bold on the image you're trying for. Just a thought.
One more thing, experiment. Low-light is ridiculously good fun. I'm working on a series right now of images taken no faster than 30 sec, all hand held. Great fun this...
This image has not been altered in PS. Only basic toning. The table is burned in over 25 secs or so, then the face of the DJ is "popped" by the flash at the end of the exposure. Same principle as described above, just taken a step further.