Stay positive. The papers may be full of despair, but letting that affect your attitude won't boost your closure rate. Strong belief will encourage your clients and give your own pitch more oomph.
Share the load. When the economy goes down, the risks go up. Discussing deals with the team will help make your offer more enticing.
Don't appear desperate. If you seem anxious to close, it gives the other party an advantage. Completing a deal in a tough market is satisfying - less so when you see the 10,000 unwanted widgets you accepted in your clamour to do so.