Really? Why? Isn't the sun moving along some path in the milky way and can't we use that to accelerate the launch of a spacecraft? It looks like Voyager 2 used the effect, at least according to this image:
For interstellar trajectories you can use stars for gravity assists, but I'm not sure that would work for escaping the Sun. There, you would want to use the Oberth maneuver, where you get into an elliptical orbit around the sun and then fire your engines at perihelion to raise your orbit to a parabolic or hyperbolic trajectory.
In any case the Voyagers both used gravity assists around the gas giants (Voyager 2 used all four, while Voyager 1 just used Jupiter and Saturn because NASA decided to swing past Titan and didn't have enough propellant left to resume the Grand Tour course).