I find I'm more inspired to do things after I watch shows like Sort Your Life Out or Marie Kondo. I'm far from perfect at this, but I tend to do each room at a time - eg all my clothes on the bed, and ask myself... Have I worn this? Does it still fit?
For me, the biggest thing was getting over how much money I had spent - and trying to shift the mindset of "oh I could sell this" or "this cost me XXX".
The money is already spent - and any money recouped is a bonus - but often the peace of mind of having it gone is better than it hanging around waiting for someone to buy it for £1. I also ask myself when decluttering: "if I need this in the future, is it easy to source and buy? Did it cost me less than £30? If the answer is yes, then I tend to get rid.
I like to use various methods from different people - Marie Kondo helped me most of all getting over the guilt of keeping something that has been bought as a gift - if I feel guilty getting rid of it, I tend to thank it (in my head) for coming into my life, acknowledge that it's job was to spark joy for the gifter in choosing it for me, and for me in receiving it. I didn't need to use it nor do I need to keep it, as it's purpose was in the gifting. This way, I can regift it or recycle, donate or sell.