Does it mean guarding your heart as in the people you surround yourself with? The things you allow in? Maybe even in a romantic way?
Guarding your heart from the people you surround yourself with can sometimes be the hardest thing. And yet for others, it can be really easy. The people you hang out with should be strong, wholesome, Christian people who encourage you to have clean thoughts and who don't try to tempt you down the wrong path. They don't say things that put down other people and they don't put you down either. Surround yourself with people who will lift you up and encourage you to seek God and do the best you can for Him. In this way, you can guard your heart.
The things you allow in... This is always difficult. But one thing to keep in mind is that we are to be in this world but not of this world. I'm someone who has been raised to avoid any places that could have something there that could not encourage me guarding my heart. Whether it be a party where alcohol and drugs are encouraged which I strongly dislike, an event where I know only people who swear and talk down on others will attend whether I participate in it or not, or even guys and girls working out in the same gym, which is something I strongly disagree with because of there being something to look at with every turn of the head. I avoid situations where I know the purity of my mind could be compromised. It is my way of guarding my heart and showing my respect for the future man I end up with. Yes, I know many do not guard their eyes anymore because they don't think it's possible in a world full of impurity. Maybe since it's around us, we should get used to it? But I, for one, will never get used to a girl showing more of her body than necessary because everyone does it. I will never get used to a guy openly looking at women and thinking it's okay because everyone does it. I will never get used to someone saying "well it's there so we have to accept it." We don't have to accept anything. I'm a strong believer in guarding my heart this way. I always have been. I always will be. We are to be in this world, not of this world. Avoid places where girls are immodestly dressed. Avoid places where a guy will think it's okay to lust after you, even if you are fully clothed. I know many of you disagree with my thoughts on this, but that is fine. I respect the fact that everyone will have different opinions on these things.
Guarding your heart in a romantic way is a big thing. I have tried my hardest to guard my heart in this way for 18 years and there are still places of my heart that I keep well guarded simply for the sake of my well being. I decided from the time that I was 14 that I would never fully give my heart to someone until the moment that God said it was okay and that it was safe to do so. I strongly encourage you to do that. We shouldn't be the ones to decide who we should lower the romantic guarding around our hearts to. That decision should be God's and His alone. He will say when the time is right. Do not awaken love before it's time. And God knows the right timing. We need to be patient and trust that God only has the best for us. All we need to do, is guard our hearts, and know that we are safe in the hands of our Father.