I’m going to do some pointing at the moon for this post, so take it for what it’s worth.
There is, in my opinion, a distinction between the soul and spirit, when talking about the individual on the microcosmic scale. I don’t believe this is how most people treat the subject, however. Most people I encounter would, I suspect, use the two words to refer to the same thing: that part of us which is eternal. For me, this is only accurate when describing the Spirit, and I’ll tell you why.
St. Paul, one of the most Gnostic of the Biblical authors, refers to the three parts of a person in 1 Thessalonians 5:23 where he says: “And the very God of peace sanctify you wholly; and I pray God your whole spirit and soul and body be preserved blameless unto the coming of our Lord Jesus Christ.”
This mirrors the tripartite division of humanity as the Valentinians would have seen it: the hylic (people of matter), psychic (people of soul), and pneumatic (people of spirit). St. Valentinus’ teachings ultimately stemmed from those of St. Paul, so this stands to reason. While, on the one hand, St. Valentinus is referring to the people (the macrocosm), and on the other St. Paul talks about the individual (the microcosm), the concepts are two sides of the same coin. Humanity and the human have three parts. 
What we call the Spirit, or Sacred Flame, is that Spark of the Divine that unites all people with the transcendent Godhead. It is the awareness of the true reality of this Spark that we call Gnosis, or at least an aspect of it. The Spirit is eternal, unchanging, and, in typical everyday consciousness, occluded by the other parts of us. The work of the Gnostic is to clear the debris of a life lived in the physical world so that the light of the Sacred Flame can shine through and illuminate the world.
What, then, is the soul, if not all that? I believe the soul is the animating principle, that which gives us Earthly life. It is our thoughts and emotions. For almost all of us, the soul is a collection of patterned responses developed as we grow up. Through instinct and conditioning from the moment we are born to the world we create a persona that allows us to interact with the material world and the people and things in it. If you believe in the archons in some form, you could say that they (or the forces they represent) have created the world in such a way as to encourage the individual to build psychic (in terms of psyche) structures that block the awareness of the Spirit. I like to use the metaphor of a mirror. It’s made of glass, but there is a coating of reflective metal, often silver or aluminium, on one side. The mirror is the soul, but behind it sits the Sacred Flame.
The mirror reflects the world back to itself. It is very good at doing so. However, through spiritual practice designed to develop awareness, we can erode the silver coating on the mirror, little by little, until the Sacred Flame shines through. When our soul is as transparent as glass, that is the state to which we aspire. We call that Theosis. Gnosis is the awareness that there is a Sacred Flame burning behind the mirror, Theosis is the light shining through a transparent soul.
 It should be pointed out that I know of no modern Gnostic groups who believe in the kind of radical determinism that the Valentinian texts imply. The three divisions of humanity are seen as states of being, and are fluid. Individuals can move between these states through spiritual work and awareness, or lack thereof.
Photo credit: Holy Spirit dove window by hickory hardscrabble