Don't bet on "systems." Instead, learn to develop a better subjective sense of the game. This means trying to build skills (which are internal), rather than relying on a system (which is external). These skills can be based on different factors - statistical analysis, instinct, or finding exploitable strategies.
Learn bankroll management. You need to decide what your bet size is going to be relative to your bankroll. This is essential to any financial endeavor where there is risk involved.
Be very careful with discipline. This might be the most important thing to learn as a gambler. You need to be disciplined with the bets you choose, the amount of money you bet, and sticking to whatever criteria you have defined for yourself.
Keep good records. You need to do this for analytics. You will be able to go over your past bets and identify patterns, and (more importantly) the reasons why you made mistakes.
Have an account at a couple of different sportsbooks (at least). You should definitely have an account at Bodog, but also at Intertops and a couple more. You should do this in order to get the best odds, but also because different sportsbooks may be better at different things (live betting, cashout wait-times, etc.).
During the playoffs you can get better subjective reads on teams. This is for a couple of reasons: first, because they are playing same team over and over; second, during the regular season, teams tend to take time off at certain points throughout the year and don't play hard. It is more difficult to make accurate predictions about how a team will play when they are not playing at their potential.
If a team is playing REALLY bad, they won't come back. I notice in basketball, whenever a team is down by 20 or so in the 3rd quarter, there seems to be a point in the game where they try to come back - either to win or simply to narrow the score to have a more respectable finish. But in games where the team is down by 25-30 points, it is usually due to a situation where a team simply did not show up to play, or is greatly out-skilled. In these games, the losing team usually doesn't narrow the score by much.
Bet against your home team often. You are giving up profits by not betting against your home team. This is because your home team is the team that you are most knowledgeable about - so you will be more likely to be profitable on those bets. Other people will give you a hard time for betting against your home team (if you live in Boston, for example the Red Sox are like a religion). Just ignore these people. Another benefit when betting against your home team is psychological: you either win money or you get to see your team win.
Bet against counter-trends. In most team sports, bet against a bad team coming off a good week, and bet on a good team coming off a bad week.
Become an expert in certain bets. For some reason, I am not good at predicting "will there be overtime?" bets. I made 2 of those bets. One was at 100-to-1 odds, and the other was at 10-to-1 odds. I bet against overtime in both bets, and lost both. The odds of this happening are 1,000-to-1.
Don't bet on "sure-things." These are bets that pay anywhere from 1.01 to 1.05. Some bettors look at bets that pay 1.05 as an almost-certain way to make a quick 5% return on their money. Instead of keeping the money in the bank for a year, they can simply bet on a sure thing. Like most extreme bets (high-odds longshots, and low-odds sure-things) the betting odds tend to be different than the statistical odds. The potential 1-5% return on your money simply is not worth the risk of losing 100% of your bet.
Become a specialist. Focus on a particular sport, team, strategy, or type of bet. Don't try to become a generalist.
Become a contrarian. Just like any competitive arena in life, success will come from identifying opportunities that other people don't see.
Concentrate on "skill bets" vs. "luck bets". Stay away from luck bets, such as whether a basketball game will end in odd or even points.
Your favorite sports may not be your most successful. Many sports gamblers love to bet on their favorite sport, but I have found that enjoying a sport doesn't necessarily mean that you are good at predicting it. The sport you are best at betting may not be the one you enjoy the most.
Don't focus on big events. Many gamblers tend to put too much emphasis on high-profile events, such as the Super Bowl, or high-profile boxing matches. In this sense, sports betting is like the stock market where so many amateurs get seduced into buying high-profile IPOs. These amateur investors believe IPOs must be some great opportunity because there is so much news being published about them. In sports betting, like the stock market, the money is made by putting money into opportunities that most people don't see.
Focus on teams you are familiar with. The teams you are most familiar with will obviously be your home team and former home team (if you lived in a different area). The other teams you will be familiar with are almost always big-market teams. Small market teams that become successful at some point (the Seahawks in football, the Trail Blazers and Golden State Warriors in basketball) may be teams that you are not familiar with. Because you rarely watch these teams play, you will not be very familiar with their style of play. There are plenty of times where a low-visibility team makes it into the playoffs, but I don't place bets on them because I simply cannot get a good read on the game.