Decide what you want to keep and what you should get rid of.
Setting up two bags is the easiest way to go about this: one garbage and another giveaways. Keep them close by, so you can add products directly into each when necessary. This also eliminates the urge to pick through small piles of giveaway or garbage products for a second or third time (where you'll likely be tempted to reconsider keeping unnecessary products) before they get into a bag.
Cardinal rule: Only unwanted makeup that hasn't been opened or expired can be given away. Otherwise, it belongs in the garbage, Reich advises.