Most sports bettors prefer to bet against the spread, which, in theory should give them about a 50% chance of picking a winner on any given straight bet.  Juice gives the sportsbook an advantage.  A healthy advantage, but not insurmountable - and certainly vulnerable to a substantial number of bettors going on a lucky streak and pocketing some big wins.

Let's look at some numbers.  If you bet 3 games a week for the entire NFL season (17 weeks) that would be 51 games.  Let's keep the math easy and say you just bet 50 games, and you flat bet $10 per game.  If you pick 27 winners and 23 losers you're up $17 at season's end.  You've picked 54%, which the binomial distribution tells us that 24% of monkeys throwing darts at a board (random guessing) could also do.  Why then do less than 5% of sports bettors make money betting NFL games?

It's not because they are bad at picking winners.  It's not because the sportsbook's juice can't be overcome - although overcoming the juice is difficult.  The real reason is that most bettors overbet their bankroll.  They bet amounts larger than they should.  Sportsbooks count on this.

What is the correct amount to bet?  That is a matter of some debate.  Let's look at an ideal situation.  Let's pretend that we can say with certainty that our ability to pick winners is 54%.  We can't possibly know this for sure based on 50 bets, but we are making an assumption that allows us to apply the Kelly Criterion to our situation.

f = (bp-q)/b

b = 10/11 or 0.9091

p = .54; and q = .46

tells us that we should be betting only 3.4% of our bankroll on each pick.  (click for more on the Kelly Criterion)

Consider that carefully.  With perfect knowledge and the ENORMOUS advantage of being able to pick 54% against the spread, you should be betting only 3.4% of your bankroll.  That's not much.  Bet more than that and your returns will drop.  Bet more than twice that (>6.8%) and you can expect to lose money even though you can pick an astounding 54% winners.

Are you committing 10% or more of your bankroll on a single bet?  The vast majority of sports bettors do.  The vast majority of them also lose money and ruin their bankrolls.