Even though league practices are heart-pounding cardio, make sure you're spending some spare time in the gym. Increasing your cardiovascular fitness as well as your strength can give you more endurance and make you less prone to injury.
Anyone who has played derby will tell you what an expensive hobby it is. No matter what your initial setup is, at some point down the road, you'll likely want to upgrade. Start by getting a new set of wheels! More than any other item, wheels will totally change how you skate. Borrow some from other girls on your league to determine the durometer and size you might like, and do some research!
Maybe you're working on Minimum Skill Requirements, or maybe you're a veteran who spends practices perfecting your four-wall or jam techniques. Wherever you are on the derby spectrum, make sure you spend some practice time working on your speed skating. Sure, most of us don't use it in gameplay, but the ability to crossover naturally will make you a better player. Many people think once they've hit their 27 laps, they're done. Instead, challenge yourself to beat your own personal record!