you could add a few muffin fans extracting air out, something diy like this link could do the trick
http://www.mobilewill.us/2012/08/diy-fume-extractor.html , only i would get 2 x 4inch fans or even bigger wont hurt, most of these can plug staight to your mains points or be powered via usb.
you could also set up something like this
http://boards.cannabis.com/diy-how-...-easy-make-yourself-active-carbon-filter.html
then add a duct externally
once you start to flower you will know if you have enough extraction as if you dont your temps and humidity will raise more daily.
you can also buy ready made if you have the money to spare. also consider a Ozone Generater, you can get usb powered ones which may help with smell.