It's a good idea to place your home on the market as far in advance as possible of purchasing a new one. If you find a new home first and then try to sell your present home, you may wind up with two mortgages. If this does happen, ask your real estate agent or banker about a bridge loan to help you make the double payments.
Lenders use the same criteria for offering bridge loans as they use for mortgages.
Shop around for the best terms. Keep in mind that when people move, sell and buy, there usually is a domino effect. Closing and moving dates have to be coordinated, and the more firmly everyone commits to a window of dates and sticks to them, the better for all involved. Put all agreements about dates in writing, and protect yourself by negotiating financial penalties for failure to comply. A home that's visually appealing and in good condition will attract potential buyers driving down the street.