The tyranny of the rocket equation usually forces incredibly efficient but slow transfers to distant celestial objects, due to substantially diminishing delta-V returns on increasing launch mass.
A common technique to get dV for "free" is to use a gravity assist manoeuver to shift a small amount of kinetic energy from a large body like a planet to the spacecraft. Small enough that it's essentially unnoticeable for the planet but a significant change in velocity for something on the scale of a spacecraft.
Not a million miles away is this concept for harpooning fast-moving but smaller objects like comets, using a strong tether and a spooled braking system to change the velocity of the hitchhiker by up to 1.5km/s with existing materials, and 10+km/s in theory.
Our experience with harpooning comets hasn't been positive so far (I'm looking at you, Philae) but with improved materials and practice this might just get off the drawing board. Certainly the cost savings in fuel might be worth the serious risks involved, and it might potentially open up many more options for transfer windows and destinations.
"Hitchhiking a celestial body is not as simple as sticking out your thumb, because it flies at an astronomical speed and it won’t stop to pick you up. Instead of a thumb, our idea is to use a harpoon and a tether," Ono said. Ono is presenting results about the concept at the American Institute of Aeronautics and Astronautics SPACE conference on September 1. A reusable tether system would replace the need for propellant for entering orbit and landing, so running out wouldn't be an issue, according to the concept design. While closely flying by the target, a spacecraft would first cast an extendable tether toward the asteroid or comet and attach itself using a harpoon attached to the tether. Next, the spacecraft would reel out the tether while applying a brake that harvests energy while the spacecraft accelerates.