Open your eyes
To get better at poker, it helps to be honest about your poker abilities. Variance can be blinding at first, but most good players have eventually realised that being honest with self criticism is important. Not harsh, not lenient. Get rid of results-oriented thinking and just be honest.
Reading up
Reading books is the most common first step. People are used to learning new things by reading books, and they give a pretty general overview and are a good introduction to the game. Lately there have been some pretty good new titles out but it's not a sufficient source in itself.
Reading online! No wai! Online poker forums like twoplustwo.com have strategy sub forums on all types of games and specific stakes. Within each one are thousands of hand history posts, where the a hand is posted, (hopefully) with all other information relevant. Other forum members can follow the hand up to the decision point, in order offer suggestions, advice, etc. This helped me improved a lot, because I started thinking about every hand I played more analytically. Essentially solving poker puzzles, similar to the way chess puzzles are constructed from a set point, helps us emulate how we'd think 'in game' thus giving practice for practical situations. Some of the most successful books of recent times, like the Harrington on Hold em series, are basically just pages and pages of hand history posts formulated just as they are online.
Training sites
Taking full advantage of the fact that it's possible to record our own computer screens, poker training sites like bluefire.com [with whom PKR have a partnership] have proved an innovative new market. Watching top online pros talk through their thought process as hands were happening was a great help to me. Watching a good/great player's cards for a whole session, following along 'as they see it' and listening to their thought process gave me a lot of confidence when I first started. These days videos are getting more sophisticated, with increased competition, and new content being guaranteed every day. They can be expensive for small bankrolls, but definitely a good investment for anyone trying to improve at poker.
Discussion with friends
Talking to friends about poker is a great way to get better. It helps to be able to talk to players better than you. No poker friends? You're probably lucky. However if you want to gain some, get involved in online poker forums, post a little, be involved. Talk to the other regulars in your game, see if they're willing to bounce some ideas around to help each other improve. Reviewing hands is very important and chatting to someone to can be the key to a fresh perspective.
Coaching?
Coaching is another viable option, but isn't recommended until a reasonably long way into development. Usually done on a 1 to 1 basis, the benefits are that you can get a much better player to give you advice and and help with hand analysis. Even just a confidence boost may prove effective enough, however it's generally quite expensive and one should be careful in selecting a coach as standards vary.
Tools of the trade
It's probably a good idea to have some knowledge of the online tools that are available. Some are free, some are not, there are some really useful items that fall into both categories. Stuff that crops to mind: database programmes to track results, odd's calculators to calculate ranges, ICM calculators to improve both SnG and MTT play. There's many others that do things like organise screen space, or auto hotkeys for actions. Look around and see what you like. When I first started playing most sites' software was not as good as it is now, and I'd be constantly finding stuff to help fix bugs or just change some colours to make them look cleaner.
There are certainly many other ways to improve at poker. The above is directed at people who will be playing mostly online. To get better at poker takes a time commitment, and improvements won't be linear, or often even correlated with results. Other important things to consider include:
Build your roll
This includes learning BR management, i.e. have a set amount of money and stick to a comfortable number of buy-ins for the stakes you play. Conservativeness is a usually a good trait here. The main thing is to always be comfortable with the stakes you're playing.
Specialise or diversify?
At the start it's probably best to specialise in one game. It's much easier to build a roll initially, and a more experienced player will be able to learn different games quicker anyway. Take advantage of reload offers, promos etc. Think about the factors that affect you when you're playing poker. This could be important to building a roll because sleepiness, tiredness, hunger can all have a huge effect on you winrate when playing.
Practice makes...
Lastly, PLAY A LOT. The more experience you get the better! Muti-tabling is a great way to increase hands per hour. I still regularly play 12+ MTTs or 9-table cash games. Compared to someone playing 1-2 tables, I'm getting a lot more experience, and opportunities to make money. Crudely put:
Money won = winrate x volume
So once you've learned how to up the winrate let's remember that volume is the other half of the battle, even if it is condensed to the last couple of sentences.
Read more from golfpro699's blog or chat with him in the PKR Forums!