Ok this will change a lot depending on what team you are and what reputation you have ect ect.
You must already have a decent tactic, or really this wont help you at all, its not going to make a tactic any better, what you see is what you get, but ill try help you make the most of your tactic.
I can only give my experence on what i have noticed in my games with a top flight club.
the club in use is arsenal.
So anyone can add some information about what they do when they come up against formations, by means to get a win, please add to the disscussion
Ok you have got your instructions all set your team is ready for the pitch you have taken on board who your going to play and made all the changes you need before going into the match. Now when you get to the pitch the first thing you do is check out there formation and you noticed "oh there not going to attack me" so you ask yourself what do i do now this isnt what i expected, this is where i would like to start a topic on how to read formations and what changes need to be made to help beat certain types of formations.
There are lots of veriables but its very easy once you get used to the AI format of play.
Some variables are
Home vs attacking vs defence vs normal vs 10 men
Away vs attacking vs defence vs normal vs 10 men
Some will be the same but behing home or away can add an extra advantage of dis-advantage.
So ill try explain what happens in diffrent situations ingame.
Remember this only will hold water for a top flight club, dont try this if your underdogs please

.
This applys for a home game vs a defensive formation, kickoff your attacking vs defence the reason you do this is because you have a home advantage you should start of positivly to try and get an early lead, if this dose not happend then you will need to change a few things, there are play deep (about 3 notches from your normal setting) change your mentality down to normal 10-11, play shorter passing 2-3, slow tempo 1, this will make your team pass the ball around more within defence and midfield (looking for the opertune moment)this will either make the other team come out to you for the ball leaving there defensive positions behind thus leaveing a gap for your team to attack into, or they wont come after the ball and you keep possesion, to the AI this is not part of the game plan, they want to win the game just as much as you do (remember its early in the game and to them its a long time to hold on without the ball) not having the ball to them is not good(at this stage in a match), so they will switch to an attacking tactic (they think "oh this guys stupid hes not going to try win" and close you down hoping to anticipate 1 of your sideways passes and then hit you with a goal (ah ha! ive got him now. but!), this sounds like a bad thing, but because your waiting for this to happend you switch back to your attacking tactic raise your def-line and raise your passing back to normal, now we have an open game (attack vs attack) and you can attack much better(remembering we have home advantage), chances are you will get a goal as you have stayed in the same formation your team is in control of the situation with only making minor changes while your opposition has changed tactics completly(just like lemmings

), this is what you dont do, but yet this is what a lot of people do and thats why you become unstable or prehaps you dont do anything and stick with your attacking tactic, most of the time i found this gets you nowhere fast. This works for most types of situations, if you get a goal and then score a second the ai switches to a defensive formation hoping to hit your attacking tactic on the brake, while its a nice thing to attack vs defence (the feeling of not being under pressure is faulse) in o6 this worked well you could brake down the AI with attacking strats, but on 07 its diffrent and the AI will cause a drop in rating's(playing at home and having all them chances missed

) then you get no goals and a drop in form which in the long run will lose you matches, so you must play a diffrent style to what you did before(but dont change formation too drasticly).
So the idea at home is to make them use there attacking formation without making drastic changes yourself, 9 times out of 10 you will score first because you have used the right game plan for this game.
For away matches they normaly always play attacking vs you you should use a normal mentality and while your defending quiet well you will be also looking for gaps in there attacking tactic, because its early in the game they will be attacking you 9 times out of 10 in my experence you wont become under that much pressure your players arnt trying to go fwd that much and so things are stable, the home team is at home they should be looking for a goal but this aint happening, so they changed to a silly all out attacking tactic (maybe a424, but it also come as other formations like a 442 with long wing arrows), so you want to use this madness at your advantage and play slightly more attacking mentality set time wasting to rarley (you should always start an away match with 7-10 time-wasting this get in the heads of the home side and frustrates them and will boost your player rating, if done correctly it wont be much but 7 instead of a 6 helps) and you should get a goal, once this happens your in control they will either go defensive in this case do the same as above for home drop back short passing slow tempo (fanny about) make them come to you, by now you should be getting the idea, what your doing it making the AI change tactics so much to get somthing out the game all this swapping and changing will do them no good and you can have a field day choosing the right time to slip in a goal, if you find your playing vs a decent side, you have just been re-ranked highly and so they play there most defensive formation, you are better off playing defensive vs them and hoping to hit them as they come to you but you might end up with a draw, in this case at least you kept a clean sheet your players got pritty good ratings for getting somthing away from home, and really the result isnt as bad as it seems, somtimes matches cant be won for love nor money, so a draw wouldnt be so back. Its at home where you can really dominate the play, because you have that home advantage.
I hope this helps, and i hope you can help me with some stratigys you make up in games to make the AI change styles, i feel this is an awsome way to make the most out of your tactic, and boost player ratings.