Hello. I have a few suggestions, but I guess I better add a little disclaimer: I am not qualified, or have taken part in any formal training, but I have researched a lot for the garden Im desining. Hope its helpful anyway!
Geometry/Form: Depending on the size of the garden, echoe shapes of areas and embrace artistic/mathmatical techniques such as the fibinacci sequence, these evolving patterns and reaped shapes in various sizes will give instant cohesion to your various areas, dont forget to think in 3D. Choose one of two lines of sight that you want to embrace (from the backdoor or seating area orcertain window for example, and have this as a guideline for the flow of shapes/patterns.
Colour: tonal colours are strong in cohesion. Let the type of soil/light quality dictate your colour scheme ie sun loving plants tend to be bright fiery hues (though nowadays with all the hybrids and intensive breeding this isnt such an issue). If your like contrasting colours choose with care. The right contrast, and echoes of this contrast in different areas will make a strong cohesion, but too cray a contrast will only srve to further break up the areas.A tonal garden I would love to see: say working with fiery coulours, at the front starting with yellows and evolving into oranges into reds into marrons and purples, contrastings drops of each colour into the colour into the colour before: drop of orange in the yellow, red in the orange, cerise into the marron and purple (these drops would work well in tandem with the last point).
Planting schemes: Depends on the look you want really, but choose plants that not only compliment one another visually but in growth requirements too. Ie an Azer that likes sun could shelter a plant that prefers/pops in a bit of shade like a bluebell or viola. If you have a patch of hostas you love plant some lettuce next to them (there are some lovely varieties like rosso). The leaves will look nice and slugs wil eat the lettuce not your hosta. Put in a few peas (dont bother training just let them romp) if a plant is hungry for nitrates as they nauturally lock it into the soil. This is more of a functional cohesion lol. If going for a natral look choose plants that naturally go together (seems a bt like stating the obvious there ha)
Architecture (combines with first point really...) if you have a certain shape of plant like a tall spiral cut hedge, or bal shaped shrub echoe this shape in different sizes throughout the garden, the eye will nautrally hop from one to the other regardless of what area they are in bringing cohesion (and can be used to loosen up the colour scheme if you are crafty!ha). Oddly if your landscaping involves squared off shapes, use ball shaped plants, or if the landscaping is circular use tall spikes of planting to break up.
I would also talk about materias but another poster has already covered that :)