At a quick glance this seems a simple act to do, 'guard your heart' ~ BUT, in the original Hebrew it is much MORE then a casual ... throw your hand up and stop things coming your way! It is a strong word (nasar), literally meaning, 'to imprison.' It conveys the idea of taking custody of something, and placing it under a watchful eye, even placing it under the care of an armed guard. THAT is some serious guarding.
When a guard is assigned his post, he is given very specific regulations, he is told his border of influence, and given the authority to enforce restraint. I appreciate how The Message, put it; "Keep vigilant watch..." The Hebrew Interlinear, actually uses 2 words, to capture the emphasis 'guard preserve.' Here, it broadens the picture, we are not only keeping a watchful eye on our heart, but also with the intent to preserve it, as to cover it, and place it in a very secure area!
The heart. Hm... small word that identifies so much: the seat of ones inner nature, the intellect, will and emotions. Interestingly it is closely related to 'nepesh' the soul.
So in short, it is up to us to guard, to place ourselves in a secure area, where a close eye can be kept on our mind, our decision making, and emotions. We are instructed in Ephesians chapter 6, to put on the armor of God, and taking up His weaponry of His word.
"Above all else..." (this is important, so DO it!) guard your heart ..." (get militant about it!) for it is the well spring of life" (the determining factor of your life).
Guard. (I know, easier said then done)