In 'urban sniping' there are many new challenges to overcome that aren't found in woodland type sniping; camouflage is much more difficult, and you have many, many, many more corners to deal with.
One of the most important skills to have is protection. You will need to be able to hold your own in a CQC environment but still stay light. Do not let your guard down while moving position to position and even once you've established a 'secure' hide in the AO. Check all your corners being prepared to have someone around each and every one of them. Moving in numbers and/or with extra rifles(additional weapon i.e. M4 to your sniper rifle) will optimize your ability to protect your squad but can and almost always will increase the chance of detection.
Claymores and boobytraps are very useful as well as other tripwire-security devices that will alert you to an enemy moving in your building. Distractions and decoys are also nice to have in place.
Various light sheets(usually black, sometimes shade of walls in the building) can be used to close off rooms for defense and to allow the shooter to sit much farther back in the building and shoot through a narrower opening that that he can manipulate without showing his movement. I love seeing people stick their guns and maybe even bodies out windows or being on a roof and silhouetting themselves; that is not how its done. Basically your best camouflage is shadows, least amount of movement possible, least amount of self visible, and getting the furthest away from the corner or window you are shooting through, around, under, or over.
When you set up in a position, make sure you know the area well and have planned out how to best defend the position if under attack and the safest route for a speedy retreat or ex-fill. Have this in mind when you are picking your location in the first place too, and don't make the common mistake of setting up in a location that others would suspect a sniper would hide; don't be obvious because the enemy is a living, breathing human being as well and will do his best to outwit HIS own opponent... which would be you in this case.
I have only played urban combat as an airsoft sniper a couple of times but I must say... It was definitely more challenging than I had anticipated.