There are two parts to serendipity, at least.
The first is recognizing that it has happened. This is not just seeing the chance occurance, but also noting your own ability to take advantage of the occurance.
The second is being honestly grateful to God, or whatever spiritual/personal system you have faith in, for that occurance taking place.
With these two recognized, you can then encourage more serendipity occurring in your immediate area. To say that it was just a freak happenstance is to throw away opportunity like money. If you want more of this to occur, one has to accept the fact that you can do something with these occurances and perhaps your wanting them was part of their happening. The other is that there is a possibility of a force much greater than your own which enables the universe to uniquely coincide to satisfy that personal want. Like any gift, where you are honestly thankful to the giver, you'll continue to recieve gifts - a positive feedback you can take control of.
There is the point that nothing comes for free. This is where your faith in the system takes place. As well, you may be asked to do certain things in certain ways. While perhaps nothing to you, more than likely the results of your action will be part of serendipity for others.
All of the above you can throw away as mere metaphysical musings if you want. But maybe, just maybe...