prepping: clear the stairs and put a thin layer of snow on your landing and in run. in the spring it may be necessary to salt your inrun. you can always add more snow to the landing if you've got a hard impact.
jump: never build your jump more than half of the height of the rail. i've found that any higher is just unnecessary, as half the height of the rail allows you to add enough pop if it's a flatter rail, and allows you to slowly shave it down to a smaller, flatter jump for steeper rails if you find it necessary. in the winter with cold dry snow, build your jump first and use copious amounts of water to get it solid. in the spring time, build it closer to the start of hitting it and salt the piss out of it. when i first started hitting urban we built big jumps too, and the only advantage is a mental thing. just don't be a pussy because it looks gay.
speed: if you're using a drop in, place it 6-10 back from the start of your jump. when building a snow drop in, try to ice it in layers and use shitloads of water to make sure it doesn't crumble (snow drop ins are going to be shorter, those are the ones you place around 6' back from the jump). if you're using a bungee, set it up so that you're letting go of the bungee 6-10' back from the jump as well. speed is your friend when you're hitting urban, because you can always slow down if necessary, but if you don't have enough speed to start with you either can't hit it or it looks like shit.
rails: dont hit aluminum, unless you can land lightly like a ninja. steel vs. painted steel, doesn't matter as the paint will chip off your first few times hitting it. if you're catching on the paint, use your ski edges like a file and chip all of the paint off manually. if you're sliding something concrete, which i would suggest only doing when it's pretty cold out, pour whater over it to ice it first.
skis: DULL YOUR EDGES UNDERFOOT FIRST. if you don't it will end in pain and agony.
most importantly, remember that hitting urban is not fun. it is countless hours of agony and defeat in exchange for 30 seconds or so of triumph and celebration. but man, those 30 seconds are sweet sometimes. if you're having fun, then you aren't trying hard enough tricks.