There are a couple things I have picked up/ learned.
1) Keep an eye on your stash you might find your buying the same color or similar in different brands
2)Get things that can be multi purpose for example your fav lipstick or cream blush can be used for the opposite, an eyeshadow could be used as a blush, an inexpensive conditioner can be use as a great shave gel/cream
3)Keeping your cosmetics sanitized/clean ( spraying powdered products with alcohol and wiping them down/dipping lipsticks in alcohol) will keep them better well passed the expiration date.
4) When tossing cosmetics only throw out things that have changed color smell or consistency
5)Lastly use coupons for drugstore items or wait to make high end purchases during the store sales and for discounted stores find out when their shipment are coming in so you get first dibs

There are other tips and tricks I know but wont make this too lengthy lol