DO: Schedule specific times to help. It will help provide structure to have someone to be accountable to for showing up to declutter.
All tips are from Dr. Robin Zasio, a psychologist featured on Hoarders. She is also the author of The Hoarder in You: How to Live a Happier, Healthier, Uncluttered Life.

DON'T: Throw out things without the person's permission. Doing so will lead to conflict and distrust. The decision about what to get rid of must ultimately lie with the person who lives in the home.

DO: Ask how you can help and let the person direct you. It's fine to offer suggestions, but let her be in charge of the process.

DON'T: Badger him or her about getting organized. People do not respond to nagging or adversarial arguments. Offer positive encouragement for things he does get rid of. If a clutter problem is persistent, it's likely that the person may be highly self-critical already. Adding your voice to the ones in his head will only make him feel more overwhelmed. Instead, praise any successes, no matter how small.

DO: Divide up chores and work together. and then set up a reward for afterward (such as a breakfast or coffee date).

Don't: Communicate your frustration nonverbally. Helping someone declutter can be challenging, but folding your arms, sighing, and eye rolling are all ways of conveying criticism. If you find yourself unable to stop letting your frustrations show, take a break.

DO: Show empathy when the person is experiencing anxiety. "I know this makes you uncomfortable. I can see why. However, I think that you can handle it and would like to propose that we try to continue to move forward."

Don't: Tell the person you’re helping what items should or shouldn’t be important. That will only prompt him to defend the items and feel that you are not looking out for his best interests. It will also result in distracting conversations that will not help to accomplish the task at hand.

DO: Be patient. The clutter didn't happen overnight and isn't going to un-happen overnight.

Don’t: Organize for the person without permission. You may mean well, but doing it for the person does not allow him to learn how to do it on his own. If new organizational behaviors are learned and practiced, it increases his potential to maintain his success.