Don't Wait for RainFake it!
This one may sound cheesy at first, but you'll be shocked at how well this works. Instead of waiting for a rainy day to shoot, take a little spray bottle with you, fill it with water, and spray the flowers with water yourself. I found a nice little spray bottle in Walgreens' beauty section (I know what you're thinking, "Walgreens has a beauty section?" Believe it or not, they do) for a couple of bucks, and it works wonders. Just a couple of quick spritzes with the spray bottle and you've got some lovely drops of water on your petals, and no one will ever know you didn't wait patiently for Mother Nature to intervene. Get a small enough bottle and you can carry it in your camera bag (empty, of course). By the way, I've used this spray bottle technique to shoot some yellow roses I bought for my wife, and by using a macro lens you'd swear I was shooting on the White House lawn after a spring shower. Try this onceyou'll become a believer.