Obviously the first step is to make sure the existing ducts are not damaged. If not then the first choice is to put in bigger ducts. If that's not possible then the next best choice is a bigger blower. As you can see in my conversation with zz, upgrading an existing furnace's blower is not likely to happen. So a new furnace with a larger tonnage blower would have to be installed. If that's not an option then you can try the booster fan. But I'm skeptical of your chances for success. Theres probably other options Im not thinking of.
The best thing to do is find a competent and honest contractor and let him evaluate it. Yes, finding one is tough. Good luck.