Thanks for checking this guide out! I can only assume you're here because you didn't bump enough frogs during your playthrough and want to get this over with as quickly as possible.

To start off, you need a mouse to do this efficiently. While a controller also works, it will yield much worse results. You'll see in a moment on why that is.

1. Make your way to any frog NPC. In this example, we'll kindly ask our favourite amphibian boss, Pepper, for assistance.
   
   UPDATE: Thanks to user Coopatroopa for pointing out that this trick can be improved by performing it on two frog NPCs simultaneously instead of just one. An example of where this can be done is in the credits (Home after beating the game).
   
   
2. Position Niko in such a way that their and Pepper's shadow are very close or just barely touching. It might take a few attempts to find the optimal spot.
   
   
   
3. Test your spot by rotating the camera using your mouse. And by rotating the camera, I mean to rotate it back and forth like there's no tomorrow.
   
   
   
4. If everything was done correctly, then you should see Pepper continuously hop (and croak, to everyone's delight) by just rotating the camera. If that's not the case, repeat step 2.
   
   
5. Every hop / croak of Pepper equates to one frog-bump. So by using your mad camera rotating skills, you can farm those 1000 bumps much faster than by walking back and forth through the same NPC, which is what I did before accidentally stumbling upon this method.
   
   
6. Save time and enjoy the rest of your day :)

And that's it! Not sure if an entire guide was necessary for this, though. But hey, I've never done one until now, so at least I can scratch this off my bucket list.

Hope to have helped! Feel free to comment and rate.
Stay fresh and have a good one.