This may sound strange, but if you wish to make money at poker, your skill at poker may be relatively insignificant compared to your money management skills. Many very skillful poker players have ended up broke. A moderate poker player with good money management skills may be more likely to bring in a steady profit.
Some tips for effective money management
Your first tip is of course to only play with money you can afford to lose. You should set yourself a bankroll - an amount of money available for you to play poker with. Unless you like living on the edge and have no dependents, this bankroll should be of such a size that whilst you wouldn't be happy to lose it, it wouldn't cause you serious problems if you did, in fact, lose it all. If you put more than this at stake, you increase your chances of going on tilt. If you do lose your bankroll, give yourself pause for reflection before reloading.
Something else of key importance is to be able to manage your bankroll in such a way as to control the risks involved in the game. Even if you are a top poker player, you are bound to experience swings in your bankroll - some days you will get runs of bad luck that you can do nothing about. If you allow these swings to become too big, you risk wiping out your bankroll altogether.
One tip is only ever to sit down at a table with a small percentage (probably no more than 10%) of you poker bankroll. Even if you play well, you must be aware that you can easily lose your entire stack in a single hand when you are badly outdrawn by someone who should never have bet or called in the first place. By only playing with 10% of your bankroll at any one table, you give yourself at least 10 bites at the cherry. This allows you make a few bad decisions and to suffer a few "bad beats" (occasions when you are outrageously unlucky) and still be able to play on. You are giving yourself a chance of making an overall profit.
Allied to this is the obvious point is that you should play for sensible stakes in relation to your bankroll. If you have a bankroll of $100, you shouldn't really sit down with more than $10 at any one table. With $10, you can comfortably sit down as a No-Limit Hold 'Em table where the blinds are 10c and 20c. This way you have fifty times the big blind and can afford to wait for a decent hand before you play. You could sit down with $10 at a table where the blinds are 50c and $1 but this would be a bad idea - the blinds would eat up your stack far too quickly for you to play sensible poker.
Another key tip (especially useful for relative beginners) that particularly applies in no-limit games is that you might be wise to get up from a table once you have made a reasonable profit. For example, if you sit down with $10, you might decide to get up and leave the table if you reach $20. You can then pocket your profit and sit down at another table with $10 again.
Not every expert agrees with this advice. They might ask why you would want to get up from a table where you are winning, but think about what might happen if you simply play on and on at the same table:
You might play excellent poker and make 19 correct decisions in a row, making yourself a decent profit. However, your twentieth decision might be wrong and you could lose your entire stack in one go, wiping out not only your profits from 19 correct decisions but the $10 you sat down with. Just as bad, you could make the correct decision but get wiped out by an outrageous bad beat.
As someone new to the game, you can't be expected to make the right decisions in those big pots 95% of the time. So, give yourself a chance - set yourself a reasonable target and cash in your profit when you reach it. This way you can afford to make regular mistakes without going broke! This strategy reduces the swings in your bankroll that might otherwise risk wiping out your bankroll entirely.
Another problem with simply staying at a table when you have made a decent profit is that you may then be playing with and risking a larger amount of money than you are comfortable with. This can then lead you to making incorrect decisions. If you would feel uncomfortable betting your entire stack even with a great hand, then it is time to leave the table and sit down elsewhere with a stack you are more comfortble with.
Another key money management tip is to set yourself maximum loses for any particular day. The key reason for this is that if you lose too much money in one day, you run a higher than usual risk of going on tilt. If you go on tilt, you might then run into real trouble, losing more money than you can comfortably afford during a increasingly desperate attempt to win "your money" back. If you stop playing for the day, you give yourself a chance to compose yourself and be in a much better state of mind when you sit down to play another day. You are then much more likely to be able to win.
Finally, I would always advise players to keep a strict record of winnings and/or losses. Keep a note of how much you have deposited at each poker site, how much you currently have in your poker room accounts and how much you have withdrawn. This way you are forced to be frank with yourself about just how much you are winning or losing. If you plan to become a pro, you can see whether you are on the right track. Even if you don't mind losing a bit playing poker in the name of entertainment, you should hope to reduce your losses as you become a better and more experienced player.