I work best when constrained. I want to have a criteria within which I can produce images. Working in this way means that I am free to concentrate on making images rather than wandering around thinking about what I am going to take pictures of. I know when I walk out of my door, exactly what I want to go and shoot - or rather, I know exactly how I'm going to shoot.
This lends itself to working in series. I think about what I want to achieve, I then set limitations and I can then go and create. That's a rather glib interpretation of that part of the process, but it is correct in it's essentials. In fact, this part of the process is much longer and more difficult than taking the actual pictures. These limitations take many forms - they can be aesthetic, practical, technical, thematic, motifs, symbolic, allegorical, subjects, geographic (e.g. northern), lights and much more.
The upshot of all this is that when you are done you have a bunch of images that all work together and can be presented as a whole. The themes and motifs that you decided on earlier will run like a thread throughout the series and the repetition of those things will give meaning to them.
It also bears mentioning that the symbols and motifs that you choose need not be obvious. If they are overt it can be off putting, so it's best to be subtle with the knife.
"Into this wild abyss,
The womb of nature and perhaps her grave,
Of neither sea, nor shore, nor air, nor fire,
But all these in their pregnant causes mixed
Confusedly, and which thus must ever fight,
Unless the almighty maker them ordain
His dark materials to create more worlds,
Into this wild abyss the wary fiend
Stood on the brink of hell and looked a while,
Pondering his voyage..."
John Milton, Paradise Lost, Book II