I have written posts about how to wash your makeup brushes before, but there are often questions about it so I thought I would write a new post about it now! :)
Also read my recent post about makeup brushes: Makeup brushes - Choose the right brushes for makeup!
How often should you wash your make-up brushes?
Brushes for cream products - I know that most people are careless with this, with makeup brushes used for creamy products such as liquid foundation and the like, they should be washed after each use! Bacteria love to use and moist makeup brushes, and even if you close your eyes to the bacteria, dirty brushes will also affect the result on your makeup as they are not as soft and nice anymore.
Brushes for dry products - Eye shadow brushes and things that are used for powder do not need to be exactly as careful, even though the best thing is of course to wash as often as possible. At least once a week, I think you should clean them!
Lazy? - A tip for you who are both lazy and hygienic - invest in several brushes instead, then you can put the dirty ones on top and take a new one every day, and then just wash them all at the same time. So I do :) I also use a cleaner that you can use to quickly wash the brushes between the thorough cleanings, and it is Laura Mercier Brush Cleanser* which both removes makeup and kills bacteria.

How do I clean in the best way?
Detergent - I only use makeup brushes in synthetic hair, and they are washed with ordinary detergent. Brushes with real hair should be treated like normal hair, with shampoo, wrapping and conditioner, but the synthetic is kind of plastic so they will never dry out or the like.
Cleaning - Wash with lukewarm water, preferably do not let the brushes soak before. Massage the brush against the palm (gently!) And rinse under the tap, repeat until you see that the foam is completely white and the brush is clean. It may be required a few times with some brushes.
Be careful with the straws, especially if you have brushes with real hair, if you rub on them too much the top can come off and then the brush is not as soft anymore! In addition, they can get messy and ugly if you are not careful, and you do not want that :)
Tips for you who wash many at the same time - I usually pour a bowl of water and detergent, then I dip the brush in the bowl, then it does not use unnecessarily much detergent.
Hard-washed brushes - If a make-up brush is difficult to get clean, for example mascara and lip make-up can be difficult, then it makes it easier to use a tiny bit of makeup remover and it comes off super easily! I use my eye makeup remover for waterproof makeup, works gallantly.
How to dry the brushes?
Press off excess water with your fingers, straighten the straws so that they do not lie messy (and do this while the brush is leaning with the straws down) and then let the brushes dry lying down! If you dry them standing, there is a great risk that the water will run down into the shaft and dissolve the glue.
If you do not have so many brushes, it can be good to dry them so that the straws do not touch anything at all, by putting them on e.g. the table edge with the straws on the outside.
Then just wait! It takes about a day to get the brushes dry, but some dry faster.
Do you have any questions, or maybe even your own tips, then just shout out :)