Choosing an advisor? Well, figure out who wants to work with you. Professor Bigshot may have a lot of connections, but if he isn't willing to take the time to read your chapters or get your letters of recommendation in on time, he won't be a whole lot of good to you.
Selecting what courses to take? You can find numerous ways to challenge yourself in graduate school while maintaining some flexibility. Join a journal reading group. Join a workshop. Go to talks in or out of your field. You don't necessarily need to enroll in astrophysics 101 because it sounds interesting. When the semester gets tough at the end of the term, you'll wish you had some flexible time in your schedule to give to your top priority.
There are numerous ways in which a graduate student can apply this adage. When you enter graduate school, the tendency is often to treat the workload much like you did when you were an undergrad. Spend a couple of weeks/months feeling out the coursework before joining the Juggling Club. If Professor Bigshot is a jerk to you now, he will probably be a jerk even after you've asked him to be your dissertation advisor. Take the path of least resistance instead, surround yourself with supportive faculty and don't opt to take on anything you can't handle.
