Rain Dance is now a long-forgotten archetype, since it tended to backfire and faster, deadlier new archetypes came out. Now, the main thing with Rain Dance is to have as little Pokémon as possible, since you need TONS of water energy.
First of all, since the Blastoise line is the most vital to the deck, you want to have a lot of it. Add some more blastoise and squirtle, take out the omanyte-omastar line, since it's not that good, and take out the seel-dewgong line, just because there's a little something else you can put in that's a real killer ever since Neo reared its wonderful head.
Add some more Magikarp (MAKE SURE IT'S THE TR MAGIKARP!) and some more Gyarados. Be careful about playing this family, though, and don't play a magikarp unless you've got a gyarados in your hand. Get rid of the Poliwrath line, since it takes too long to power up to and you don't want more than two evolutions in a deck. And for the real killer-- Wooper. This little Pokémon may not seem great at first, but it stall the heck out of any electrics and can slam for a while, too. The total anti-Rocket's Zapdos.
As for the trainers, Make sure you've got a lot of breeders -- they're needed for that turn 2 Blastoise, and maybe some traders. Add Comp Search and some Oaks, and you're set. The addition of Energy Flow-PokéMon Center can help you heal anyone who's wounded and being able to play all that energy back down with Blastoise is deadly, as well. Don't forget about the abundance of water energy, as well.
Here's how the deck looks now:
PokéMon(15)
3 Squirtle
1 Wartortle
3 Blastoise
3 Magikarp(TR)
3 Gyarados(Base 2)
2 Wooper
Trainers(15)
3 Oaks
3 Breeders
3 Comp Search
2 Energy Flow
2 PKMN Center
2 Trader
Energy(30)
30 Water Energy
