All the organizational methods in the world won't help if you just don't have enough space.
I live in a little apartment, and no matter how many pieces of dual-purpose storage furniture I buy or how strategically I plan out how to store things I have a bunch of, I can't overcome the fact that I just don't have a lot of room. I have a storage unit (and have for years), and before you say that you don't need one, consider the following.
When I got my first storage unit, I didn't know what it would be like at all. All I knew about storage units was that the ones I'd seen back in my rural hometown were massive, meant to hold farm equipment or old cars, and the ones I saw on reality shows about locker auctions were also huge, for the most part. All I wanted back then was someplace to store my stuff for a few weeks while I was moving apartments, so when I showed up to the storage place and found out they had plenty of closet-sized units, I was both surprised and grateful.
All I really need is a closet-sized one, since closet space is what I'm missing in my home. I store out-of-season items in mine, so right now it's full of hoodies, coats, and boots, but in a few weeks, I'll get those back and put my shorts, swimsuits, and beach things in there. I still have room for other items, like an air purifier I never use, and valuables I don't need to see but do need to keep.
What you could put in there depends on a few personal factors, like where you live and what you own. Other good ideas include:
Holiday decorations
Off-season outdoor tools, like lawnmowers or snow blowers (in bigger units)
Bikes, skateboards, snowboards, or other weather-specific outdoor equipment
Special occasion items, like fancy dresses or fine china
Any household goods you buy in bulk, as long as getting to the unit is relatively convenient
I've written a lot about how to organize storage rooms and garages, but if you don't have one of those or those spaces just aren't that big, no amount of shelving or hacks will really fix it.
I used to use a storage unit that was self-serve. It was pretty useful for me, but it had limitations. Because I don't have a car and live in a big city, I couldn't put larger items in there. I couldn't easily get them in and out. I found a different company that picks up my storage items, brings them to the unit (which I'm not allowed to visit in-person), and brings them back when I ask. I pay $80 per month for it and to me, it's extremely worth it, but it does require some planning, as I need to tell them a day or two in advance if I need something out of there.
You can also start small. Pick a smaller, less expensive unit, then upgrade to something bigger if you need it. If you go in-person, you can usually work out some kind of deal on the bigger one just by haggling a little with the worker; that's what I did when I upgraded from closet- to bigger-closet-sized after two years of the mini unit.
I do recommend using organization tools within the unit, too. Just because you're using an out-of-sight, out-of-mind approach to organizing your home doesn't mean the things you stash away should fall into disarray or be hard to access when you go get them. Stick to the organizational rules you employ in your home, storing similar items together and making sure they all have a designated spot. I made this mistake in the first unit I rented, allowing all the clothes I stuck in there to turn into a jumble that got worse and worse every time I stopped in to swap new ones in. I don't need to organize now, since I use a company that does that for me, but when I upgraded to a bigger unit, I got a clothing rack and some shelves, which really revolutionized the thing. A shelving unit can cost as little as $17 and storage containers that keep moisture, bugs, and dust off your stuff while keeping it all organized by category are invaluable (but typically you can get four for $31).
