Wash yourself mid-ride. I discovered this little trick on a hot 400K ride. About 200 km into the ride I was starting to feel a bit of chafing so I went into a toilet, using the paper towel and water from the sink, I washed my sitting area, and dried myself off well. I was fine for the rest of the ride.
I've done that on rides since with much better results than any time I tried using lubrication of any sort ... and I tested a lot of different creams etc. in 2002 and 2003. However, if it is raining a light layer of some sort of cream might help protect the skin.
On rides over 300K, I'd bring a spare pair of shorts ... wash yourself, dry yourself, and change into a fresh pair of shorts.