Fight for the best mortgage rate possible. Period.
Mortgage rates affect your payments more proportionately than prepayments. If you are considering purchasing a new home in a good resell area like Montgomery County, Maryland or McLean, VA, you probably will have equity by the time you close on the home.
Also use online real property databases to see the value of homes in the area. Your realtor will more than likely give you a comparative analysis report, but you can do that on your own right now. If a new development offers a home at one price that is higher than what you see on the search, parlay that into your initial offer.
I don't live in that area, so I can't say with any certainty, but that's HIGHLY unlikely in this environment.
Banks have the final say in your value and if your home is considerably less than others in the neighborhood, yours doesn't appreciate to their level, the OTHER homes decrease to your home's value..i.e a foreclosure.
Banks are now SUPER-conservative in valuations.