If you haven't cleaned you makeup brushes within the past 2 weeks, or ever, then they are in desperate need of a bath. Typically, most experts suggest cleaning them weekly, but if you tend to forget or just think that's not accurate, then try to wash them at least every 2 weeks. It will only take a few minutes to wash them all, and if you do it at bedtime they will be nice and dry for use in the morning.
Begin by gathering your makeup brushes, two clean hand towels, and whatever shampoo you have on hand. Bring everything to your sink and lay it out. Keep in mind that you will be washing your brushes, but even if they have natural bristles you will not be using conditioner, only shampoo.