Emotional Capital - But How Does One Increase The Amount That They Have?

Increasing one's emotional capital is achieved through the accumulation of value (in the form of meaning) generated by continuous emotional exchanges that become retained and co-owned by both parties due to their positive net worth.  

Each time we engage with a someone, we should aim to ensure that the net sentiment each party takes away leaves them feeling good, or at the very least, better than they were before. If both parties agree that the sentiment each departs with had a positive net effect upon them, than it can be assumed said sentiment possesses value, in the form of meaning.  

Positive sentiments generated in this way are then retained in the memories of both parties as they are meaningful which coalesce over time, leading to the eventual formation of a good rapport: a complex of active opportunities for engagement either party may tap into to instigate an interaction.
  

Each opportunity for engagement stems from a shared knowing (a single interaction both parties found meaningful and thus have consigned to memory). This shared knowing functions as a narrative node when the two parties subsequently meet thereafter (a narrative node being a point from which future stories can be written or shared together in many directions) and affords greater fluidity in communication due to the invoking effect a shared knowing has on both party's desire to create new, or continue on from pre-existing narratives. 

In this way emotional capital can be increased.  

In short. Be nice to each other.