Speak to your sales representative and start your new home search as soon as your existing home hits the market.
If you've found a home, before you've sold your existing one, use "sale of your existing home" as a condition on your offer. If you don't sell your house within a fixed period of time, you can choose not to go through with the offer. This, however, is a difficult condition for many vendors to agree upon and you may find that you have to forgo your price negotiating power.
Purchasing a home before you sell could be a risky strategy if you're counting on the proceeds from the sale.
If you've found a purchaser before you've found your next home, use "purchase of a new home" as a condition when you sign back the agreement. Again, it will only be for a fixed time. Even if you have not found the ideal next house by the time the deal closes, you may still wish to proceed with the offer. As a buyer with a "sold house" you will be in a better position to negotiate price.