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.
Step 2: Remove the Screen Window and Case
Once the window is off you can unscrew the FIVE screws (black one in the center) and pry the case off. Again, watch out for the screen.
Step 3: Get the Circuit Board Out.
Step 4: Find the Inductor and Antenna Wire
Bypass the inductor (small black box)
Lengthen the Antenna
By taking a short, 2cm length of wire I had laying around I was able to noticeably increase the transmission power. I sat in my car, tuned to a fuzzy station that had a little bit of some station coming through. I tuned the Belkin to the same station and started playing some music - very fuzzy, cutting in and out with the local station. When inductor was bypassed as shown in the last picture, all I could hear was MY music, and much clearer. I then used the same 2cm piece of wire and touched it to the free end of the antenna - this also had the same effect.
Step 5: (semi) Permanent Modifications
The next step is to desolder the antenna wire and replace it with a longer one. You can use any length you want, as any increased length will be useful. A quarter wavelength antenna would be ideal. This length is about 76cm:
c / f = wavelength
c = 3E10 cm/s
f = ~ (88+108)/2 = 98 MHz (average FM frequency)
--> wavelength = 306cm
--> 1/f wavelength = 76.4 cm
If you usually use just one or two stations near eachother, you can fine tune this length to be specific to those frequencies, but I doubt such small adjustments will make a noticeable difference.
Pictures / more steps to come (soldering this evening)