There are a couple of really easy things you can do to boost the transmission power. It seems to make a pretty significant difference to the reception.
The thing is a little tricky to tear down, so I'll post some photos of that process.
Here we go...
*** Disclaimer: Boosting the output power may cause your transmitter to exceed FCC regulations on unlicensed FM transmitters. Neither Instructables nor myself assume any responsibility for your actions; this instructable is provided for learning purposes. You should accurately test the output power of your transmitter to make sure it is within regulation or you may incur a fine or other penalty.
Step 1: Remove the buttons
The up/down tune buttons are secured to the face with adhesive. You just have to pry up the buttons and deal with the fact that you might have to reapply adhesive. These need to be removed since there's a screw underneath.