You will probably bug in as that is where you and your gear, job, contacts and resources are, at least until it becomes obvious the disaster warrants a bugout. Depending on your resources and attitude to risk you should prepare to hunker down for two weeks to two years.
Ideally you want everyone near you prepped so you can bring them in with you and leave their homes empty as a perimeter.
Get out of areas with hardcore religion, lots of youngsters with no resources or slums with transient, lazy poor types who will create chaos. If you must stay urban then look for low crime skilled Anglophonic healthy unpolluted areas with education and entrepreneurs who can trade or think their way out trouble, but do not assume they do not have links with crime lords or government (or government with gangsters). Contrary to the advice of many preppers to stay urban for resources, research into civil war shows that tends to kill due to choosing the wrong people as your future.
Stay away from, or plan for, chemical plants and nuclear power stations. Stay away from targets like ports, major roads and airports.
From research in Chechnya we know that if you end up fighting an urban war then half will end up depressed and a quarter psychotic, typically weak, worried or warlike. And that is without knowing your world has ended in a global catastrophe.
Your so-called friends may feel like allies as they cooperate while they are safe, but once there are not enough resources for all of you then you will realise they were only ever aides, who will eventually transition through neutral, to becomes obstacles by siphoning off your resources and ultimately going hostile when they decide to at least gradually phase you out. Without security you do not have shelter, medicine, food or water – somebody more aggressive does, or will.
Learn from the art of war the importance of strength, speed and secrecy. Do not get tired, do not let anyone know what you have, but be able to repel anyone who comes after you anyway, and do not let them know you have that strength and thus something worth protecting unless they already know what you have or are coming on spec anyway, in which case still have more strength than you reveal.
Choose a building that is reinforced with a concealed escape route.
Ideally:
barricade ground floor openings,
cover higher openings to stop items being lobbed in,
fireproof wooden floors with at least a couple of inches of sand,
dot fire blankets and buckets of water around for firefighting,
hide antennas.
An army would smash, shoot or blow their way in, but more realistically you could stop most looters by reinforcing against normal tools, bricks and arson. Whilst reinforcing against a civil war scenario like guns, grenades and gas is feasible, it is expensive, destructive and probably too late to start once trouble flares. Remember, in urban environments you cannot see or radio far, and attackers can sneak from all directions.
If concealment is the priority then choose somewhere hidden, away from lines of drift, where you can observe and escape from, sheltered and with communications ability.
Limey Prepper - UK Preppers Handbook 