If you are not prepared to be able to survive a post grid down situation in an urban environment, then you have no other choice but to take resources and materials to be able to survive. try to scavenge first from the public space rather than the private. Any sort of building that is funded by our tax dollars might have resourceful materials.
Instead of going straight to the corporate sector for utilites and food where hundreds of people will think of going first, you may think of going to manufacturing and proccesing warehouses. These places are full of materials that are yet to be distributed to the corporate sector. You can bulk up on your resources before people start raiding these places. You will have to be careful though since the security of these sites will probably be on high alert.
These places will probably have heavy machinery that you can use to help protect your shelter. These sites will probably have materials that you can use to create barricades. Other things you can get from construction sites are generators and possibly some kind of fuel source.
Try to find shelter where you will be able to easily see people heading towards you rather than be blocked by trees and high buildings. Also find shelter that is made out of strong properties and hard to get into. Your shelter should be fortified enough to be able to take hits from guns,molotovs, flamethrowes etc. If you can, try to make your place look resourceless and a waste of time for people to try to scavenge.
Use signs to your advantage to foster indecisiveness on looters and scavengers. Some good signs are "looters will be shot dead" or "looters will be sniped". It doesn't have to be violence oriented either. You could post a sign saying "danger: hazardous chemicals spilled" or "nuclear waste inside".
When the grid is down, cell towers are probably not going to be operable, but if you are a survival tech person you can probably do something with these cell towers. Cell towers are meant to be the highest point wherever they are located. You can use this to your advantage by climbing to the top and surveilling your shelter area. Try to stay as camouflaged as you can, and only go up if you are properly clothed to be able to go up with relatively low danger.
The first thing that will hinder the average person is not having access to the internet for knowledge. The best places for vital information are schools,universites,libraries and any book store. Having information that explains how to make useful materials is vital for long-term survival.
Almost everyone will be going to stores to get bottled water, and it will be gone in a few days. Meanwhile you can be collecting gallons from local swimming pools, house pools, and indoor pools.If you run out of water a good water purifier could be used to drink water from ponds lakes or even rivers.
Once the grid is down one of the most vital resources are medical and toiletry materials. Try to scavenge for defibrillators, med kits, toilet paper, soap, fire extinguishers, etc. Once a major disaster happens people die mostly because of disease and infections and not having the proper medical attention to wounds. Try to stay as clean and safe as possible. Know whats the limit of your body to make sure you don't get injured.
Solar panels can be used to power up electronics that you may have. Try to buy phone chargers that charge using solar power. try to get as many products that you can that can be charged solarly. Use batteries as contingency once you can't draw power using the sun.Batteries will be hard to come by as its one of the materials most people will scavenge for first.
Survival training is not just useful for dealing with manmade or natural disasters. It's actually quite useful for different activities such as mountain climbing, nature trekking, etc.
In terms of sustaining the human body, nothing is more essential than water. The body can go for long periods of time without large quantities of calories; however, it cannot survive for long without water.
One of the biggest challenges in creating a sustainable disaster kit is finding food that will stay edible and nutritious for long periods of time. Of course, there is always the option of purchasing canned food, preserves and other commercial food products.
First aid is the preliminary care given to injured individuals to reduce the chances of permanent damage and to bridge the period between the actual injury and the point wherein proper medical care can be given to the patient.