A subject near and dear to my heart.
Advice:
1) Pay attention to the lifestyles of professors. You'll hear a lot of professors talk about fascinating work. That stuff is cool, but it does not pay the bills. You'll see who is wearing expensive clothes (pay attention to the shoes above all else). You'll see there are some professors that regularly mention things that cost a lot of money (e.g., expensive vacations, second houses, boats, airplanes). You'll also see professors that are clearly struggling more than they should. Use that to guide what you are pursuing.
2) Do NOT take advice from professors that are financially unsuccessful. Don't be a jerk to them. Listen. But don't take their advice as it relates to money.
3) Learn the history of clinical psychology, beyond whatever that stupid textbook is. IMO, there is a tendency for the new subspecialities in psychology to offer greater income. As specialties become less new, the income seems to go down (e.g., psychoanalysts were absolute ballers in the 1940s, neuropsych is newer and does well, forensics is newer and does well, RxP is like 15 years old and pays very well, etc). Use that to guide your decisions.
4) Learn about payor sources. Any idiot can look up the CPT code for psychology stuff on a CMS website, and multiply that code to see how much you can gross (not net). Once you learn about this, you can keep an estimated running tally in your head of what you would bring in during practica or internship. DO NOT SAY THIS TO SUPERVISORS. It will piss them off.
5) Develop relationships with professors, and ask for career advice. It's an easy way to suck up. The more information you have about that, the easier it is to evaluate others' advice, and the easier it is to make decisions.
6)Kids affect things. IME, many, but not all, female psychologists who decide to have children work fewer hours over the course of a year. I would recommend keeping this in mind when getting advice. I would also recommend considering this factor if you decide to have children. It's important to have kids and raise them well. But it does affect income. In short, you can't compare the productivity of someone who can work until 10pm with someone who has to leave the office at 2:30pm to pick their kids up.
7) When something comes across your plate, ALWAYS ask yourself what the reward is. It could be non-financial, but it must have a point. For example, there is basically no way that writing a book or chapter is worth the lost income. However, it may help in marketing how awesome you are.
8) Consider where you are in your career. If you're 65, have tons of cash, a paid off house, etc... well you can volunteer for stuff. If you're 30, and have $300 in your account... that's not a time you can afford to give away your work.
9) I would HIGHLY recommend reading practice books. There's not many out there. They're easy to read.
10) Listservs are an incredibly way to learn. Free. Put it on digest. Read it once a week.
11) Google various psychologists to see what they do.
12) When you learn something, ask yourself, "How can I apply this to making money?", "Who would pay for this?", "Who CAN pay for this?", etc.
