I bought the BU-303 to replace an ebay special that proved far from waterproof, and this time took the time to waterproof it myself.  After waterproofing with this procedure, my receiver has survived some very heavy rainstorms, both on the road and parked, as well as me washing the car and not being very careful around it.  I realize from other peoples' horror stories that when not weatherproofed, receivers tend to corrode internally over time, and thus not fail for weeks or months, so I can't be totally sure this procedure works perfectly, however I know for sure that it's an improvement over just leaving it stock, and if it dies on me at any point it will just be an excuse to buy a Sirf Star III unit.

First off, flip it over and peel off the rubber pad, exposing two screws:

Take out the screws, and then snap the case open: 

I applied silicone everywhere there would be a seam, ie- around the entire edge of each half, liberally around the cord strain relief, and I removed the little plastic LED lens from the side, squirted silicone into its hole, and put it back in. 

After that, just put it back together, and all the excess silicone comes squirting out:

Just wipe it all off and you're done.  I let mine dry for a day or two before I actually used it outside; I think the silicone should still do a decent job keeping water out while it's not entirely dry, but I wanted to play it safe.

 Update:

A few weeks after doing this, my BU-303 stopped working.  I would plug it in, and it would sometimes come up with 'USB device not recognized', and sometimes it would recognize it as the virtual com port, but it wouldn't give any serial output, and the LED would only have the faintest glow to it.  I opened it up again (which wasn't easy after siliconing it all, I ended up mangling the plastic around the seam a bit) and found out that the capacitor (the thing that looks like a small battery) had died; it was corroded looking underneath.  It's a problem I had heard of other people having; I don't know what caused it, whether it wasn't as waterproof as I thought, or if the capacitor just died on its own - there was no visible moisture or other corrosion to suggest that water had gotten into it, so I am leaning toward the capacitor being crappy.  Since I was away from home at the time, the best I could do was just tear it off.  I'm not 100% sure what purpose it ever served, but the receiver works just fine without it.  Unfortunately, after being taken apart too many times, I also broke the screw posts in the case, so I can't screw it securely shut, and therefore I no longer trust it to be waterproof. 

So, I have it on my dash under the windshield for now, and I consider it a good excuse to buy myself a BU-353, which will be a far superior performer, as well as being waterproof without modification.