You really just need to practice and be consistent, and eventually it'll get a bit easier because your vocal cords will change a bit. I also recommend practicing varying your pitch often when you're by yourself. Go low, go high, find your range. Make it all muscle memory, and you'll naturally find the extremes. Singing is also good exercise for it, whether you can or not. What helps most is exercising your vocal range so that you can better control your voice, and this will also help it from awkwardly cracking or squeaking when you're around the extreme ends of your range.