To get the biggest amount of bolts in the game you must...

1. Beat the game once.

2. Get all the sewer crystals, but dont spend them right away.

3.Go around through the sewers and demolish those green slime creatures until your bolt multipler is at maximum.

4.Finally, while your bolt multiplyer is at it's fullest, go to the plumber and give'em the crystals.

(After steps 1-3, it would help if you find a bolt multiplyer box)