Make sure you measure yourself around the waist at the point where you plan to wear your jeans. Try not to rely on other the sizes of other clothing brand sizes, since you don't want your brand spankin' new pair of jeans to either choke the shit out of you or hang off your ass.
Be particularly careful when you're buying raw denim, or when you're venturing into new brand territory. Each brand sizes differently, and it's always a good look to ask someone working at the store how the denim is expected to stretch and conform as you wear it over time. Remember, raw denim will stretch as it wears on your body, whereas washed denim will likely maintain its shape from the date of purchase, or even shrink slightly. Keep these things in mind as you browse different stores and selections, and don't be scared to ask about each pair's unique evolution.