There are specific terms Pikachu will understand when it's fishing. Say these words to encourage Pikachu. The words to say are in red, as always. =P

reel it in - If the Pokemon is small, just say reel it in. Reel it also works, too, plus it's shorter.
pull - Say this when the Pokemon is very close to you. Pull it will also help.
release it - Some Pokemon are way too big to reel in, so you may want Pikachu to save its energy for something a tad smaller.
pull harder - Pikachu will pull on its rod harder, strengthening the chances of catching the hooked Pokemon.
hang on - If the Pokemon's pretty big, encourage it to keep the Pokemon on the line.
hang in there - Works almost the same as hang on. Use these two phrases first if the Pokemon looks huge.

If the bubble bounces right off Pikachu, tough luck because that means it won't listen to you. Try encouraging from the start to keep this from happening. Also, it's best to say words that will reel in the Pokemon whenever Pikachu suddenly turns red itself. When it turns red, it means it's strong.