Your team acquires more experience points every time they play in both a league or a friendly match. The formation experience grows in proportion to the number of minutes spent in that formations. A team gets extra experience sublevel each average 10 minutes, the smallest expected time block, at lower levels (after Excellent experience grows slower).
Poor is the lowest possible level of formation experience. The base level to which everything automatically moves to is Poor [middle]. You need at least two full matches (usually friendlies) to avoid confusion.
Remember that routine grows in block of 10 minutes. So for example:
if you play a certain formation for 30 minutes, you get 3 extra blocks of formation experience. If you play a certain formation for 90 minutes, you get 9 extra blocks of formation experience.
At the start of your thirth game, the formation experience you already got to Excellent [block 2], will fall to Excellent [block 1].
At the start of your thirth game, the formation experience you already got to Formidable [block 3], will fall to Formidable [block 2].
The highest 3 levels (Excellent, Formidable and Outstanding) are actually 'double levels'. This is confirmed by HT's. Each 10 minutes you play a certain formation, it will rise one block (at low levels that results with 1/3 of a level, at high levels 1/6 of a level).
Due to the selling of a player it can go lower though. So anytime you sell a player, you slightly risk losing formation experience.
Furthermore, if you don't play with a certain line-up for some time, the experience in that formation will decrease (it can happen only during one of the two weekly match updates).
Each game you play, all formations fall 1/3 of a low level (Poor, Weak, Inadequate, Passable, Solid) or 1/6 of a high level (Excellent, Formidable, Outstanding).
|Each game all formations lose one block of experience (except the ones that are on Poor middle). This happens also to the formation you start your game with.
For this example, at the start of your second game, the formation experience you already got to Passable low, will fall to Inadequate high.
A good idea is to always use an alternative, or switch between at least two alternatives. If you use more than three alternatives it will be difficult to maintain formation experience.
Concurrently the 4-4-2 is no longer the "standard" formations, so it does not maintains excellent routine. Any legal formation, being no more a subset of 4-4-2, gains formation experience in the same way starting with season 42 (April 2010).
Before the decision to ban extra players (central defenders, inner midfielders, or forwards) and some types of extreme formations, some line-ups couldn't get any experiences. For example, until season 38 (February 2009), national teams and National U20 teams would get formation experience even when using four or more inner midfielders or central defenders, but this was removed just a couple of weeks before the start of the World Cup.