The three key ingredients for my work (time and ideas notwithstanding) are canvas, paint and loads and loads of brushes of all shapes and sizes. From expensive natural hog hair to cheap hardware store synthetic ... I have (and use) them all!
Shapes & styles
The most common shapes of brushes are pictured below – I mostly switch between using round and flat (square) brushes. Around 80% of my paintings are done using flat tipped brushes because I like the straight edges and exacting lines of paint they leave behind, and angular curves they can create. Round brushes are nice for softening shapes and, of course, the little 'dots' of light that often appear in my skies.
The other style of brush I'm using more and more is called a paddle brush. It’s super short-handled and makes me feel so connected to the canvas. I have these brushes in several different widths, the only drawback being that all are too wide to simply dip into a jar of paint.
Different type of paint, different brush
Bristles for oil paints need to be thicker and stronger to push the denser paint around and withstand the additives and cleaning solvents. Acrylic paint is easier on brushes and there's a wider range of synthetic or natural brushes that can be used. Watercolour brushes are really soft and typically not used elsewhere, but in all honesty, my absolute favourite brush at the moment is a 1" short-handled watercolour brush. Once again, I'm not one for following the rules!
I'd probably spend more on brushes if I treated them better but I'm terribly lazy about washing them properly. On any given day my studio sink is filled with pots of water and handfuls of soaking brushes.
I'm a bit better about cleaning the brushes I use for oil paint even though the process is time consuming and messy (wiping, soaking, rinsing in mineral spirits, wiping, soaking, rinsing etc.)
If my brushes are lucky, once a month i'll take the time to soap them up properly and even condition the bristles with a brush restorer/conditioner. I should do this more often because it really can extend the life of a brush and it's noticeably more pleasant to paint the morning after my brushes have gone to the spa!