This is possibly the single most useful bit of practical Stoic advice I've come across.
Let death and exile and every other thing which appears dreadful be daily before your eyes; but most of all death: and you will never think of anything mean nor will you desire anything extravagantly.
As I mentioned in misconceptions, focusing on the bad is a stereotypical, and misunderstood, Stoic trait. As this parts makes clear, the idea is to use your awareness of death to help you appreciate what is truely valuable in life.
This negative pre-visualisation can be confronting and uncomfortable. My own death is not something the greatly worries me, it never has been. Not that I'm reckless (far from it), but the idea of dying doesn't disturb me. That could be because I haven't fully accepted it, I'm not sure. In any case, thinking about the death of people I love is much more effective in changing my behaviour.
When I meditate on the inevitable death of my wife and my sons, it makes it incredibly hard to get cross with them. It drastically increases my patience, slows my emotional reactions, and allows me to find that space between action and reaction.
It is a hard line to walk, between honest realisation and needless worry. As Seneca says, we do more harm to ourselves by worrying about things than by experiencing them. To absolutely act as though my kids would die in the near future would be paralysing, I think. But the acceptance that it will happen, and that I cannot control when, is empowering. It pulls me in to the present moment and helps me to appreciate what I have. It's sometimes overwhelming, in the best of ways.