It's been said, pull the boot, clean the drain hole. You can spray inside the boot and the top of the plug with WD-40 (its original public consumer use was as a water displacer for ignitions).
Dielectric grease on the boot is OK, but I recommend a Hole Shot Fenda Extenda as a permanent cure for puddles or rain.
yeah... the fender or the rad flap will help... the only thing is with the fender you can still get fling off from the road so it can still get in the plug area...
so by applying the non conductive grease on the plug boot it helps seal the plug so if it does get into the area it won't make it in to the end of the plug as easily... keeping the drain clear is important... most older bikes have this all plugged up with street crud
I like to combine methods... I never had a problem with it on my 99 but the caps were super tight so it sealed well even without the grease... I couldn't really even get the caps off very easy... the drain was clear but I did notice a lot of crap got flung into the plug area... the advantage is I pull into a parking garage at my condo and any water in this area has a chance to dry up well before I shut it down... and my new bike has the flap
but it couldn't hurt to have a little backup...
I wonder if a 2nd gen flap thingy could be rigged to work on a 1st gen... any one try it?
one other good thing about the grease... when it comes time to pull the plugs, the boots slide off easier...