Making the right move

Getting started in the real estate market is easy enough. All you have to do is look for the right place, put your offer in, get it accepted, set up the closing date, and then give your landlord the necessary notice. Or if you're buying a business, you don't have to immediately give up your day job when you open the doors. You can wait until the sales are coming in sufficiently.
Getting out of the market isn't difficult, either. You can simply get your place listed and move out of it to wherever else you choose to go. And, assuming that this is going to be a rental, the temporary cost on top of any mortgage payments is no doubt managable while you're waiting to get your existing place sold.

But if you're moving up or down in the market or planning to move across town or to a different community, you're faced with a question to which there is no immediately obvious answer.
Do you buy and then sell? Or is it better to sell and then buy?
In the first case, you can end up with two places. In the second, you may find yourself without somewhere to live!
Naturally, a great deal is going to depend on your circumstances and upon whether the market is likely to be kind instead of unkind to you. Whatever else can be said about the business of buying and selling houses, it doesn't come with a whole lot of guarantees!
Nevertheless, there are some steps you can take to minimize the risks.

Offer conditions
There are two commonly used "conditional upon" clauses used in Agreements of Purchase and Sale that can take some of the pressure off you.
One allows you to sell your existing property before making the deal firm. Many vendors realize that a couple of months may be needed for you to find a buyer and will recognize that you won't have the money to close until you do.
Admittedly, this Sale of Purchaser's Property (SOPP) clause usually permits the vendor to consider and even accept other offers in the interim. However, you then have the option of removing the condition, which you may feel confident enough to do, given your by-now sense of how well the market is going to take care of you.
Into the bargain, if you've given the people you're buying from a quick sale, it's quite possible that the market will give you one of your own as well. Moreover, most agents -- although unfortunately not all! -- will steer potential buyers away from a SOPP property. It only invites disappointment when the house they like can turn out not to be available after all.
At the same time, though, a determining factor is certainly the likelihood of your present house getting sold. If you already have it listed, it helps. If it's priced correctly, there's less uncertainty still.
At the least, however, the clause does protect you from unexpectedly ending up with two houses.
You can also incorporate a "relocation" clause when you accept an offer on your own home. In fact, it does no harm to give warning of it in the wording of the listing agreement -- and you'll perhaps have seen the phrase in various MLS listings.
Anyway, the idea is to give yourself time for finding something else and it's generally not seen as a disincentive by people who give you an offer. The general view is that if you're sure of getting the money, you can afford to put up at a motel or rent an apartment for a short while if there's nothing immediately available for you and you have to wait until it turns up.
Nevertheless, potential buyers do need to know how serious you are about moving. Obviously, they aren't going to be too interested if they think that maybe (but only maybe) you'll start looking for something once you have an offer on your place. It will therefore help if there's evidence that you already have an idea of where you're headed.
In any case, with these two clause ideas you can, as it were, work both ends against the middle. Neither of them makes for absolute certainty, but they do reduce the chance of your ending up with all or nothing!

Previewing the market
The sensible step when you're interested in making a move is to get a sufficient measure of the current market on both sides of the equation ahead of time. How well are houses like yours selling and at what sort of prices? How many homes are available where you want to go and how long do they remain available?
If the answers are the right ones, there's no real reason why you couldn't/shouldn't embark on the buy/sell process as soon as you're ready for it. It also probably doesn't matter all that much whether you start by buying or begin by selling, especially if you incorporate SOPP and Relocation clauses in whatever offers you make or receive. Moreover, you can in fact opt out as soon as it suits you, because both clauses contain a phrase along the lines of "included for the sole benefit of the (buyer or seller, whichever applies) and may be waived by him/her at any time."
Along the way, it's a good idea to share your intentions with the real estate salespeople you contact. On the listing side, you need them to give you a good enough indication of what your house will sell for and how quickly. On the buying side, you need them to at least point out half a dozen or more potentially suitable homes and/or locations, along with some relevant MLS statistics that tell you the purchase prices and length of time on the market.
If things are running in your favour, these agents will have some built-in means of then keeping in touch with you. It doesn't have to involve an ongoing transmission of new listings, which is often simply a waste of everyone's time, particularly if your move is going to be several months or longer away. However, there's a mutual benefit if you can be kept apprised of the market and made aware of any significant changes in it.
If nothing else, you can hold a watching brief of your own by periodically checking the ads and, if it exists, the local real estate board's public MLS database.

Short term financing
The idea of a bridge loan seems to no longer belong in a banker's portfolio, but the principle can still sometimes come to your rescue. If your income is sufficient and especially if your present equity is high enough, it isn't impossible to get a mortgage that, for just a few months, covers you for both the house you want to buy and the one you intend to sell. All that's required is a provision that lets you pay off the amount set against the value of your present home as soon as you get it sold.
Something similar can often also be arranged with the builder if you're interested in a new home. Or you may well be able to merely pay a deposit and leave the possession date open until your place is sold.
Moreover, there's nothing to stop you -- save for lack of the nerve it takes! -- from asking the vendor to hold a mortgage until your own sale closes. Naturally, it will depend on whether the vendor is also in a sell-then-buy situation, but it can happen. Although the business of buying and selling houses doesn't come with a lot of guarantees, it does have its share of nice surprises.

The main point, whatever your case happens to be, is that you needn't become a hapless victim of Murphy's Law when you decide to change your address. There are ways whereby you can minimize the consequences.