1031 Exchange

http://en.wikipedia.org/wiki/1031_exchange
or google

tax benefit for investors that allows you to DEFER taxes on non primary residence properties and take profits to be used towards another LIKE PROPERTY purchase for investment use. Gets you out of paying capital gains on investment property, for time being and use that profit for next purchase.

LIKE property is buying a new property at or above what you sold the last one for. There is a timeline to purchase as well, but I think its a gray area and as long as you show intent to buy you can qualify. Ask your accountant about that part

I dont think it has to be at or above the price to be considered "likeproperty", just that you will be taxed on any amount you make that you do not rollover.