Start when you first move in to the new unit! That's right, one of the keys to getting your deposit back later on is to document the condition of the rental unit when you took possession of it. I advise that you do three things:
1.) Complete the following checklist and give a copy to your landlord. Some landlords already do this (a good sign), but for those who don't offer it, be proactive and offer it to them. Let them know that if they dispute any of the items they are welcome to come in and look for themselves. Here's a link to one such checklist:
2.) Take pictures with a friend as a witness. Be sure to point out any defects to your friend while doing so. This should be someone willing to go to court if needed. (Maybe you can have a reciprocal agreement to go to court for each other.) If you have friends come over during moving day, it can't hurt to point these flaws out, especially any major ones.
3.)Store the photos and the checklist in a safe place with your rental agreement and any other paperwork or notices you ever receive from your landlord. During your tenancy, any time you have an interaction with your landlord regarding the state of repair or other issues be sure to write an account of the conversation and put it in the same file or drawer.
Be sure you also maintain the same records for previous rentals and keep any pictures that you took of other rental units in the past. If you end up in small claims court, don't be surprised if your landlords contact past landlords listed in your original rental application. You may need this information to provide a rebuttal to their testimony.
I will continue this topic as I have time.