Decluttering is the highest return on investment, when it comes to getting and staying organized. Sadly, clutter is a leading culprit in our sense of overwhelm, day to day stress, and yes, even fights with those we live with and significant others.
For most people, getting rid of sentimental clutter is the hardest thing. This clutter challenge is normally accompanied by guilt and sometimes even shame, a reason why holding onto the objects was "important" in the first place. When it comes to sentimental items, try creating a legacy box, book, and/or timeline with loved ones, if possible. I love creating what I call "Decade Books" using Chatbooks (this link will get you $10 off your first order). They are an affordable options to aid in letting go of the hundreds of boxes of loose photos, and transforming them into a conversational handheld legacy. I also use these for yearly memory books and special trips. Next up are those items that you spent a lot of money on. I take the adversarial position and say to let it go instead of trying to sell it. BUT, if you're going to try to sell it, give yourself a timeline. If it doesn't sell within it (say 3 months), know where it is going. From a psychological perspective, you are more likely to continue to let things go, that you don't need or use, if you simply let them go instead of holding on (which gives you an easy out to go grab it from the garage before you even list it). Remember that the money you spent is not wasted, and if donated, will go to a great cause. Try searching locally to find an organization that matches your values, it will make it easier to know it is going to a good home.
The last "tricky" item I'll cover here are clothes. Specifically, I find this true for women that are either between sizes or perhaps entering a new season of life. First off, fluctuating weight is normal and nothing to be ashamed of. If you are entering a new field of work, are a new mother, bodies and clothing needs will change. For work clothes, a rule of thumb is based on your laundry schedule. If you wash clothes once a week, or once every other week, having two weeks of clothes should be plenty to keep things fresh. For fluctuating weight or sizes, keep current size, and at max one down (if you are working to lose weight). Keeping more than one size difference will just bog you down and can be more harmful in maintaining a healthy body image. Remember that all bodies are beautiful. For the extremists out there, you can research a capsule wardrobe. I personally find this to be too restrictive, but instead, I use a "seasonal" box. I swap out about ⅓ to ½ of my wardrobe twice a year (for summer and fall transitions in California).
With these three tricky decluttering tips and tricks under your belt, you can set your timer and get started on one area that you want to prioritize in your decluttering and later organizing efforts. Needing more support, try a course, virtual, or in-home session with a a local Professional Organizer.