I would recommend reading the thread on "Listening to Katrina". It has a lot of good info on what you need for a good bug-out-bag. In general, you should consider where you live and what you will need to bug out from. Natural disasters like snow/ice storms, hurricanes, tornados, earth quakes, volcanic eruption, etc. Man made disasters like terrorist attacks, industrial accidents (at a fixed location such as a plant OR along major roads/railways such as chemical spills/releases). Also, accidents, like your house catching on fire. Listening to Katrina has recommendations for all of these scenarios based upon how much time you have to get out to safety and how far you need to go. For instance, if your house is burning down you aren't going to have time to collect your worldly belongings, but if your bug-out-bag (BOB) is on your way out of the house along with your keys, cell phone, and wallet/ID, then your BOB should have the following: a change of clothes (including shoes) and copies of all important information, including home insurance, car title/registration/insurance, birth certificate and/or passport, possibly a backup of family pictures stored on a flash drive, possibly your pistol/CCW plus ammo depending on the laws where you live.
If you have a hurricane barring down on you, generally you will have some warning of its arrival. You will have the BOB built for the "in case of need for immediate evacuation of the home", and you can layer beyond that of what additional supplies to take if you are getting out. Depending on what types of events you may encounter where you live, you can develop general plans for what you need to take with you to get out. Maybe you will have time to pack all of your guns and ammo in the car before getting out, but there are other things that should likely be packed first. A well designed BOB should be the first thing to be packed.
Also, consider your abilities when designing your BOB. If you amass a collection of wilderness survival gear (tents, axes, saws, etc), then make sure you are comfortable and proficient at using such gear. For example, I am not a camper. I don't enjoy sleeping outside or living off of the land. In fact, I've never had to do it, and I don't regularly put myself in situations where I would NEED to do it. I am, for all purposes, a "city boy", and as a result I don't keep that kind of gear on hand because I don't anticipate needing NOR would I know what to do with it if the need were to arise. Maybe that is a fault on my part, but it is reality. If you are good at the whole outdoor, camping gig, then use it to your advantage.
Remember, it is really easy to get "spun up" on the whole bug-out concept and planning for various scenarios. I am an engineer, and when evaluating the safety of a design we say "only design for one credible event". This means don't think "What if the hurricane coming through causes the nuke plant to melt down and spreads fallout over half the country? I need to be able to make it to Canada without stopping for gas." You can plan for melt down and the hurricane, but don't plan for both happening at the same time. Also, think about the most likely events when planning your BOB and strategies to get out. Ignor the less likely (but often more interesting) events. For instance, if you live in a small town in Mississippi on the gulf coast, you need to be planning for hurricanes of various magnitudes, not a freak snow storm or a terrorist attack.
Finally, remember that your BOB shouldn't be designed to provide for survival during and after TEOTWAWKI. In truth, no BOB can provide for such an event, and the likelyhood of such an event occuring is so low, you should seriously ask yourself the question as to whether it is worth planning for. If your answer is yes, then you need more than a BOB.
