Thank you all for your reply.
There is nothing wrong with the GFI itself, and I don't think it's the lights either, as others are having the same problem (2 other homes). What we have to do is, when it rains and the breaker trips, we pull a cord from the garage (not on a GFI) and plug everything into that, everything works fine,no breaker trip. The GFI seems to be too sensitive for connections such as Christmas lights and cords that are across-the-ground. Who in the heck strings their cords off the ground? I tried taping plastic bags around the cord connections, but this just accumulated and trapped water inside the bags and made things worse.
I hate GFI's, and don't know of ANYONE who has electrocuted themselves from an outside outlet - compared to the number of people that I know that have a nightmare with their Christmas lights going off when it rains! Again, the breaker only trips when it's raining, no other time. I guess I'll just not be able to use my outside outlets for Christmas lights (STUPID). I think this is equivilent to the banning incandescant LIGHT BULBS and GUN CONTROL.
Me, I'm probably going to take the dang GFI's off of my outside outlet circuits - so that I can USE them.
Thanks all for the replies!