It depends on the goals you are losing; are they headers, or a through ball down the center, or just silly dumb mistakes like losing the ball next to your own goal, or stuff like that?
If you are losing in headers, you can: a)get tall center backs. b) try and stop their wings to cross the ball in. If they are in the wing, use your full back to block the path of the cross. Don't try to go in for the tackle unless you think you have a very good chance of getting the ball.
If you are losing via through balls, then try to anticipate at the balls they play and get ready. If they have a 1vs1, definitely do not try and tackle them unless necessary. Try and stay about 1metre (in the game, not real measurements!) from him. Always try to get your player between the opposition player and the goal, therefore denying him any chance for a shot on goal. If you do lose your position, and the opponent looks like he's going to shoot, then use slide tackle to tackle the ball. This is risky, and should be used as a last resort.
If you keep doing silly mistakes, try to not dribble as much and clear and pass the ball more. Also just stay calm and don't rush into the tackle. Try to be more conservative and not just launch your player at the opposition.
As for the formation, I recommend the Man Utd formation, as some others have already said (it pains me as an Arsenal fan to tell you this). But i prefer to put the left side midfield more inwards, almost like a AMF. Also, being an Arsenal fan, i like to use the Bergkamp-Henry attack, where Bergkamp does the passing, while Henry moves between the Left and and center. As for defending, try to use a back four that pushes up the pitch; you can do this by making back line to "A" and offside line to "C" (I know that most recommend to use "C" and "C"). The reasons for this are simple. For a start, when the defenders play with a "C" back line, a more withdrawn forward (SS) can exploit 'the hole' position. The hole position is from between the line of defense and line of midfield.
------GK-------
---DEFENSE----
-----'hole'-----
---MIDFIELD---
This makes it that if a CB goes to mark him, it will pull him out of position, but if he is left free, it leaves him unmarked. But if the defense line is pushed up, this hole will become much smaller, meaning its not very useful.
Another good side is that you will win the ball further upfield, so that you won't always recieve the ball in a danger zone (area close to the goal). And even if someone can break through the defense, there's still enough time to recover with a fast defender.
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
My current modified 'best' team:
--------------Cech---------------
------stam-----------lucio--------
zanitti----------------------A.Cole
-----------------Essien-----------
---------Gerrard------------------
-----------------------Rosicky----
-Eto'o----------------------------
------------Bergkamp-------------
-------------------Henry---------