You are basicly stuck to the RAM consumption of your fields if you can't do anyting to the field content.
One consideration to do is to make sure you load your qvd containing the data optimized in your final application. I have not laborated so much on this my self, but @rwunderlich wrote a blogpost on the topic back in 2008. It can be worth your while - QlikView Cookbook - When less data means more RAM .
