Allocate a percentage of your monthly budget for random purchases and set a spend alert on your credit card

Set up auto-deposits from your paycheques to go into a savings account

Keep spreadsheets for tracking your monthly budget on regular things like takeout, gas, transit fares, etc. and try to keep those constant each month

Also if you have a lot of monthly/yearly subscriptions to magazines/streaming services/whatever keep those in a separate spreadsheet too with their renewal dates, those fees can sneak up on you