But by using the natural attributes of the huge masses dotted around the Solar System, you can do the job quite easily - by using the planets as slingshots to speed your craft on its way.
Imagine a stationary planet alone in space. A fast-moving spacecraft approaching it is attracted by its gravity and speeds up (with respect to the planet), flies past (because its momentum keeps it from being captured), slows down again as it is pulled back by the planet's gravity, and then continues on its way. Net change in forward speed, measured as distance covered over time: zero.
But the spacecraft has changed course because of the slingshot effect caused by the planet's gravity. It is now flying in a different direction from that it was on before - but without using a drop of fuel. It's a bit like grabbing hold of a handrail as you run down a flight of stairs to turn yourself quickly to reach the next flight of stairs heading down.
This process was essential for the Ulysses spacecraft, which used a slingshot around the planet Jupiter to change its flight path out of the plane of the ecliptic (the plane in which the Earth and planets orbit around the Sun) into a polar orbit around the Sun. No spacecraft could carry enough propellants to do this with a rocket engine firing alone.
So change of direction can be important, but a more effective use of gravitational slingshots was made by Voyager and Galileo (among others) to pick up speed and reach their destinations sooner.
The key point here is that by using slingshots around planets they picked up speed with respect to the Sun - that is, moved faster in their orbits around the Sun. Why? Because the example above, with the "stationary" planet, is misleading. The planets are moving - they are orbiting the Sun. So while there may be no speed gain with respect to the planet, there is when viewed from the Sun. If you fly past the planet ahead of it in its orbit you lose speed. If you fly past the planet behind it in its orbit, then you gain speed.
Of course, nothing in the universe is free. When the spacecraft gains speed, the planet loses an infinitesimal amount of angular momentum (orbital speed), and so orbits just a tiny bit closer to the Sun as a result. It has given angular momentum to the spacecraft - which speeds up, climbs a bit further out of the Sun's "gravity well" and so reaches its destination sooner.