NFL writer Mark Kirwan's take on American Football Betting
Two basic principles guide my NFL betting approach. Firstly, what are the best reasons not to take one side of a bet over the other in a given game? And two, what's the best story I can tell to justify taking a side?
Calculating Yards Per Play
The most straight-forward way of answering both these questions for an NFL Sunday is to have your own sense of what handicaps should be and to compare those against the market.
Bettors will have their own way of factoring in different metrics in compiling their power rankings for this purpose, but one of the most common starting points is net yards per play differential.
To get this, you divide the number of yards an offence has gained by the number of plays they've run, and do the same for yards allowed by their defence. Subtract the defensive figure from the offensive figure and you've got the net yards per play.
Doing this for all 32 teams will give you a rudimentary power ranking of each team that will also allow you to put a figure on the gap in quality between any two teams and generate your own handicap.

For instance, last season's NFC champions San Francisco 49ers had an offensive YPP figure of 6.0, and a defensive number of 4.7, giving them a net rating of +1.3. Their division rivals the Arizona Cardinals were rated 5.5 offensively and 6.0 defensively, meaning they'd net negative rating of -0.5.
If the two of these teams were to play on a neutral field the Niners would be expected to gain 1.8 more yards per play throughout the game. For betting purposes, punters often ascribe a 1-point value to every 0.15 difference between two teams, meaning the handicap for a game between these two teams should be 12 points based on these basic assumptions.
That means, if San Fran is favoured by more than 12 points by a bookie, the value side is the Cards, and if they're favoured by less than a dozen, the Niners are the value.
The advantage of calculating your own handicap is that it gives you a dispassionate assessment of the relative strength of teams, so that you can then compare your number to the those released by bookies or set by punters on the Exchange. If a team has a huge win one week, you won't get carried away by it, or if they suffer a shock reverse, you won't lose perspective and necessarily go against them.
Other indicators
Of course, this is only a starting point. You shouldn't bet blindly based on YPP numbers. If the market line is way off your line, you should ask why that is? What's the story other bettors are telling themselves to move a market in one team's favour or another?
You can use other metrics and information to adjust your power rankings and the handicaps it generates.
For instance, homefield advantage is often ascribed a 3-point rating, notwithstanding the decline in its importance Mike referenced above.
Similarly, it is always worth trying to factor in things like expected weather - cold, wintry conditions favour defences - extended travel (i.e., multiple weeks in a row playing away from home add up in terms of fatigue, rest, preparation, etc) and rest time between games.
In recent years, the NFL has gone to a schedule where there is a game every Thursday for the first 16 weeks of the season. This means teams are often playing on shortened prep time or have 10 days between games, like a mini bye-week. That should be considered when considering a spread.
Also, it's often worth fading teams who are coming off big "emotional" wins, such as comeback wins over divisional rivals and prime time games (ie, Sunday night and Monday night games). The odds are they won't be as up for their next game, but the market may overvalue them based on seeing them play impressively in big TV games.

It's also worth mentioning injuries. Though stars get a lot of attention, it's often tempting to overvalue the impact of particlar injuries. Sure, we know that the drop from, say Russell Wilson to Seattle's back-up QB will be huge - it could be as much as ten-points on a handicap, maybe even two-TDs - but losing DK Metcalf or Chris Carson isn't quite the same in Seattle. Someone else will step in.
What is worth looking at is the length of a team's injury report. Losing a non-QB player can slow down a team for sure, but they'll usually be able to adjust. Losing three guys along your offensive line in one week, plus your running back and both safeties for a game will have an affect though, and you should try to factor that into reading a game.
Ascribing a value to different, contextual factors such as these will help you draw up sharper lines and avoid the pitfalls that more casual punters fall into based on looking at who "the good teams" are and simply picking them to cover.