Spearheading the sustainable energy movement. Attempting to solve rush hour traffic. Exploring the unknown regions of outer space. Responding to fans (and critics) on social media. To say billionaire entrepreneur Elon Musk is keeping busy would surely be an understatement.

Nevertheless, Musk took some time out of his monster schedule to share updates on his latest projects at the most recent TED conference--and in the process he shared some brilliant career advice.

Speaking on the subject of space exploration, Musk explained why he's invested so much time and money in SpaceX (his space transport company), despite the fact that so many view it as a distraction.

Musk traced back the recent steps of the U.S. space program, which he essentially described as heading in the wrong direction. He then said the following:

People are mistaken when they think that technology just automatically improves. It does not automatically improve. It only improves if a lot of people work very hard to make it better, and actually it will, I think, by itself degrade, actually.

You look at great civilizations like Ancient Egypt, and they were able to make the pyramids, and they forgot how to do that. And then the Romans, they built these incredible aqueducts. They forgot how to do it.

We could sum up Musk's point in a single sentence:

If you're not progressing, you're regressing; so, keep moving forward.

Why this is great advice.

It doesn't really matter what your personal goals are. The key to success in any field or endeavor is to keep moving forward.

A progressive mentality doesn't mean that you'll never experience major setbacks, or even utter failure--which can deliver vital lessons and invaluable experience. (Just ask Musk, or any successful entrepreneur, how many times they've gotten it wrong before getting it right.) Additionally, reflecting on how far you've come can provide necessary motivation.

But there's danger in keeping focused on the rear-view mirror, so to speak.

It eventually leads to a crash.

To maintain a forward-moving mentality, you must:

When it comes to continually moving forward, Musk walks the walk.

Like when he recently marked a major achievement by SpaceX--not by sitting back and basking in the glory, but instead by making sure the next challenge was clearly defined.

Or, when a customer complained about Tesla's breakneck pace for innovation, and Musk tweeted this.

Remember, there are no shortcuts. True success is about hard work and resilience--the ability to keep getting up when you're tempted to throw in the towel.

Because if you're not moving forward, you're headed in the wrong direction.