So here is how you do it:
1. Create an account: it doesn't take much of your time.
2. Add the button: trust me, this is your best friend. You can go to the online store and just add the item you want or bought to your Polyvore.
3. Go to your closet and try to find the item you own on Polyvore: if its a new thing just go to the store website and add it using the button. Some items may be more difficult to find, so just try to find something that looks more like your item.
4. Organize your Items: I organize my items by 4 categories - Own (for things I bought), Beauty (for beauty related items), Wishlist and Others (for any other random item)
some items in my Polyvore
So as you can see, keeping track of what you own is not that hard, and it helps a lot when it comes to getting inspiration and mixing and matching your pieces without having to try them all. Hope you enjoy, and just have fun with it!