Every time I put Ditto and Manaphy in the day care they always seem to not like playing together very much. Is there a way to make it quicker?
Edit: Specific for all games, but would prefer for Pokemon Pearl.
Nope.
The Oval Charm was already mentioned, but it was introduced in B2W2. B2W2 forward, you can use this to increase the chance that an egg is found. In earlier games, you simply wait.
The chance of any two Pokémon producing an Egg is determined by their species (two Bulbasaur are the same species, while Bulbasaur and Ivysaur are not) and their Original Trainers. In Generation III or later, the Day-Care Man will describe the two Pokémon's compatibility when the player speaks to him. An Egg may be generated after every 256 steps the player takes.
(Also applies to Dittos)
Source