We get inundated with documents and things in life. Some of us are better than others, either we are better at staying uncluttered, or we are better at finding new places to keep new stuff that "needs" to be kept.
Either way, there's a fundemental recognition that there are some things that should be kept. Here's a list of documents and information that you should have on file some where accessible to at least one other person who will know what to do with it (even if that means to give it to someone who will handle it).
Keep a list of important names and contact information: your lawyer, your accountant, your financial advisor, your insurance agent, the boat house manager where you keep your boat or the airport manager where you have your airplane, etc. Name, address, telephone, fax, email and any other relevant info (directions to get there, perhaps).
Keep your will and other estate plan documents (trust agreement, health care power of attorney, living will, durable power of attorney for finances, etc.) in a safe place.
Be sure your health care power and living will is on file with your current primary physician and any health facility you use or expect to use in the future.
Insurance policies: health, life, property. If you have a house, have the policy in a safe place; the same goes for any assets you have that are insured. Life policies need to be easily accessible when the time comes for them to be handled.
Real estate and other titles to property. (car, boats, etc.)
Safe deposit location and key should be in a secure place.
Marriage certificates, birth certificates, military papers, naturalization documents, divorce documents, separation agreement, shared custody papers, adoption documents, etc.
Passports, social security cards and records, etc.
List out all your bank accounts, investment accounts, retirement savings, etc. Need account numbers and contact information for person to help manage the account.
There are probably a few things I have not mentioned here. In short, consider anything you have in your life that has value. Certainly all your property and assets, but also, your family members, and their health, safety and welfare. Keep in a safe place all the information and documents that are in any way related to these things and people. It will be a much easier process to handle life's events when the necessary documents and information is available to you when you need it.