Someone screwed up.
All of us at some point have played behind a less-than-ideal back four. Perhaps the centre backs don't win headers, or maybe the fullbacks show their men inside and lunge recklessly into challenges. I revisited a very old interview I conducted with goalkeeper coach extraordinaire Alex Welsh for many of the points that follow, so credit (or blame) should go to him:
Buy his book, too.
First, look in the mirror. Are you the perfect goalkeeper? If not, don't expect your defenders to be perfect. This doesn't mean you can't expect more from them, but a reality check is always a good start.
Next, understand their specific limitations. Are they physical, psychological, or both? Are they simply too short and too slow? Do they lack confidence or football intelligence? Do they make poor decisions and take bad angles?
You can help with each of these issues. Let's start with the physical limitations. If you have defenders who aren't the best at winning balls in the air, then guess who needs to come help them? Goalkeepers massively over-complicate coming for crosses. Remember two things: 1) You don't always have to catch or even cleanly punch every cross. Sometimes just getting a touch, or indeed your very presence challenging for the ball, will be enough to put off a striker. Try to catch or punch, of course, but know that you don't need to be perfect to succeed. 2) You can use your hands. Nobody else can. The ball is there to be won - go get it.
Slow defenders need help with positioning and angles. I see fullbacks who get beat, then chase directly after their man, instead of taking an angle to the near post. Make sure they understand that once their man gets behind them, they need to take an angle that prevents the wide player from simply carrying the ball right at goal. By taking an angle they can make up the distance lost, and have a chance to cut out a shot or cross. They should be getting this coaching from the manager, of course, but in the heat of a game, a simple shout of "angle!" can remind him what he needs to do.
By far the biggest impact you can have on a dodgy back four is through organization. There are some very basic principles of sound defensive play that even the most casual pub team can follow. It starts with the centre backs. Keep them together. Nothing good happens when one of them wanders forward into midfield and the other drifts wide. Of course there are times when one will have to go cover for a fullback, or step into midfield to win a tackle. But the other needs to see this and react. The more they work like a two-headed unit, the harder your team will be to break down. You can use psychology here too: treat them like a partnership, a pairing, and they'll begin to see themselves that way, will communicate more with each other and react better to each other's movement.
Fullbacks are, by nature, somewhat isolated, but this can make it simpler to keep them properly positioned and aware of their responsibilities. "Keep him wide" or "show him outside" are the two most common instructions I give them, along with Rio Ferdinand's favorite, "stay on your feet." All too often, defenders feel they aren't doing their job if they aren't tackling and winning the ball. Make sure they understand that channeling players into non-threatening positions on the pitch is more than good enough.
The back four does not play in isolation. Midfielders and even strikers are critical to any team's defending, but they won't be as easy to communicate with. It's more up to the defenders themselves to make sure the midfielders understand and fulfill their defensive responsibilities. It's not about bossing people around, but simply making sure everyone knows what's expected of them.
It helps to know the individual personalities of your teammates, too. If a player is struggling, trust me, he knows it. If he's the type to respond to a verbal kick up the backside, then give it to him. If he needs a gentler touch, then have a quick encouraging word in his ear at a corner. Exploding in anger at an already fragile player is not going to make anything better. Unless you're playing at the full professional level, your first obligation to your teammates is to behave decently. Peter Schmeichel berated his defenders for 15 years, but he was Peter Schmeichel.
And we're not.
Finally, be aware of what's going on during the game with your back four. If they're getting a runaround and you get the ball in your hands, slow the game down a bit. We all like to keep possession if possible, but use your head: don't roll the ball out to a fullback who's just been beaten twice and booked for a foul - he's already feeling the heat and doesn't need the responsibility of carrying the ball out of the back. If there's one specific opposition player causing problems, it's your job to let everyone know where he is and what run he's making.
Most of this is common sense, but remember that what seems obvious now is easy to forget when something goes wrong in a game and the red mist descends. How you react in those moments is how you define your character as a player. More importantly, keeping your head gives you a chance to prevent bad things from happening in the first place. You are there to help your defenders, and, let's face it, some need it more than others.