With city living often comes the regular hunt for a cheaper or bigger place. I recently had to move from a three bedroom place to a teeny-tiny 400 sq. ft. studio, requiring some serious downsizing and consolidation. Here's how I compressed my whole life to fit into a U-Haul cargo van.
First off, I want to mention that I'm single. This helps considerably when it comes to consolidating and reducing possessions, but the basic principle can be applied to a couple or a family: ridding of extraneous "stuff". And taking advantage of being single, I alone can decide to cut down to the absolute necessary, which meant getting rid of large furniture items that I once shared with roommates.
Craigslist is your friend. If you know you're moving ahead of time, start selling your stuff well before your move date. Reselling not only allow someone else to take your stuff for you, it also puts cash in your pocket (perhaps offsetting some of those moving costs which undoubtedly arise). Craigslist is full of people hunting for bargains, and as long as you put out a fair offer, there will be no shortage of buyers. My roommates and I got rid of most of our living room furnitures within weeks, and we didn't even have to pay someone to take it away for us!
Donate what you can't sell. Learning from past moving expereince, I've kept my wardrobe to a minimum, with only a few quality items. If you haven't been as successful keeping your clothing collection to a minimum, then it's time to donate. All those sweaters and jeans that you've outgrown, the shirts and pants that have been buried for years, they can all be donated for a good cause.
Store what you can't/won't sell. Some pieces of furniture or items have more than a monetary value to you. Maybe that amoire in your bedroom has been in the family for generations. For these truly meaningful treasured belongings, you can rent a store your items in a local storage facility. Before you store anything, make sure that the facility is secure, practices proper maintenance, and offers environmental conditions ideal for your property. The last thing you want is have mold growing on your family heirloom.
Moving is not the most fun thing to do, by keeping your possessions in check even when you're not anticipating a move is always a good practice. You won't only be prepared when it comes time to move, you'll be saving money and creating less waste as well!