I have a few ideas for you.
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I agree with others, if you are going to be mixing a quantity of paint that will end up being more than say 4 or 5 brush strokes, don’t mix it with your brush, instead use a palette knife. (I recommend a liquitex #14 small palette knife - something with a little flexibility in the metal). If you mix with a brush, you can end up wasting half the paint in the brush that you then have to wipe out.
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Choosing the right brush can actually help. For example if I am only painting a small area, I would probably choose a short flat brush (often called ‘brights’) such as a Robert Simmons Titanium #6, rather than a long flat brush. A long flat brush is going to take up more paint than a short flat brush - you might not need all that paint but you are still going to ultimately achieve the same effect. Also try not to load your brush all the way up into the ferrule unless you need to.
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You don’t necessarily need to wash your brushes at the end of every single painting session. If you know you are going to be painting with them the next day, either put the brushes in a zip lock bag and / or give the brush a good dip in linseed oil. Then when you come to use them the next day, simply wipe off the linseed oil.
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I agree that keeping a few brushes on the go at a time for can help with color changes. Although I also agree that sometimes having more than one color on your brush can often be beneficial. It’s good to experiment.
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There’s a good thread with lots of advice on how to clean your brushes when you need to. Many people (including myself) do not use solvent or thinner:
http://arttalk.dailypaintworks.com/t/cleaning-oil-painting-brushes/2914