Aside: Poker and rationality aren't close to excellently correlated. (Poker and math is a stronger bond.) Poker players tend to be very good at probabilities, but their personal lives can show a striking lack of rationality.
To the point: I don't play poker online because it's illegal in the US. I play live four days a year in Las Vegas. (I did play more in the past.)
I'm significantly up. I am reasonably sure I could make a living wage playing poker professionally. Unfortunately, the benefits package isn't very good, I like my current job, and I am too old to play the 16-hour days of my youth.
General tips: Play a lot. To the extent that you can, keep track of your results. You need surprisingly large sample sizes to determine whether your really a winner unless you have a signature performance. (If you win three 70-person tournaments in a row, you are better than that class of player.) No-limit hold-'em (my game of choice) is a game where you can win or lose based on a luck a lot of the time. Skill will win out over very long periods of time, but don't get too cocky or depressed over a few days' work.
Try to keep track of those things you did that were wrong at the time. If you got all your chips in pre-flop with AA, you were right even if someone else hits something and those chips are now gone. This is the first-order approximation.
Play a lot, and try to get better. If you are regularly losing over a significant period of time, you are doing something wrong. Do not blame the stupid players for making random results. (That is a sign of the permaloser.)
Know the pot math. Know that all money in the pot is the same; your pot-money amount doesn't matter. Determine your goals: Do you want to fish-hunt (find weak games, kill them) or are you playing for some different goal? Maybe it's more fun to play stronger players. Plus, you can better faster against stronger players, if you have enough money.
Finally, don't be a jerk. Poker players are generally decent humans at the table in my experience. Being a jerk is unpleasant, and people will be gunning for you. It is almost always easier to take someone's money when they are not fully focused on beating you. Also, it's nicer. Don't (in live games) slow-roll, give lessons, chirp at people, bark at the dealer, or any of that. Poker is a fun hobby.
Excellent post. Thank you for the detailed response.
Right now, I have been struggling with calculating pot odds and implied odds. I grasp what they are conceptually, but actually calculating them has been a bust thus far. Is there any guidence you could give with this?
As far as legality in the US, I am playing in the state of Delaware with one of thier licensed sites, so I think I am in the clear. The play is very thin though, and I am looking to make my way to the brick and mortars in Alantic City to see if it will be a good sandbox to become better.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.