1) Have Multiple Outs
They say the closest thing to a free lunch in traditional investing is diversification. Similarly, the closest thing to a free lunch in sports investing is to have multiple places to wager. You need 52.4% winners to overcome the usual bookmaker spreads. Shopping for the best line can overcome some of the advantages bookmakers have over the public.
When you walk into a Nevada sportsbook you see parley cards everywhere. Bookmakers love parlays. These stack the odds against players looking for lottery-like payouts. If you have a strong, reliable edge, round-robin 2 team parlays make sense for a small part of one's wagering. But that is not how most people use them. If I were once again managing a sports betting syndicate I might carefully add small 2-team parlays to our regular bets. But the average player should avoid parlays. The same is true of prop bets. Teasers, however, are a different story. They can offer an added edge in certain situations.
If the line has been going in your direction (this often happens with underdogs), you may want to wait until soon before game time to put in those plays. If the line has been moving against you (usually with favorites), you may want to put in those plays right away. You can see which way the line has been moving (and also compare lines from multiple books) at www.sportsbookreview.com. We prefer to wager early even though we play many underdogs. This is because betting lines are generally efficient and often move toward their true value.
We are often on the same side as bookmakers who are happy to go against public sentiment. This means we often play on bad teams versus good ones. Sometimes you need to hold your nose when you bet your system plays.
Keeping good records will tell you what you do best. We look for modest edges by identifying factors (usually behavioral) that the linemakers and public overlook. These can swing the odds in our favor enough to make a profit. We do best in football, hockey, basketball, and soccer because these are more team play sports. So individual performance variances even out, and situational factors have more importance. Variances at the team level also even out in totals plays. We do particularly well with these.
Our plays are quantitative and rules-based. Much of the injury uncertainty is already incorporated in the betting lines. Sometimes though injuries to key players can overcome a system's advantage, and we consider that information.
You can find the key players in each sport at FoxSports. In the NFL, for example, go to www.foxsports.com/nfl/stats. Besides the top players in each sport, offensive linemen in the NFL are very important. You may want to pass on a game with injuries to key players or to more than one offensive lineman. Two good injury reports are here and here.
On point-spread plays, it is unrealistic to expect more than 60% winners over the long run. You can do very well averaging in the high 50% area. Legendary sports better Lem Banker became a multi-millionaire with around 57% winners.