Below are a number of basic game tactics to help you in FIFA 08. You need to have some sort of structure in your play if you expect to win games. You can't just go out and pass around and shoot when ever, you need a planed form of attack and defence.
The Possession Game
If you possess the ball, your opponent does not, and therefore he cannot score. By possessing the ball with a series of smart passes, you can run down the clock if you are winning, or wait until you see the killer passing option through to your strikers and play them in on goal. If you can possess the ball and pass well, you don't have to be good at much else to win games in FIFA 08. Most concepts in FIFA are similar to real-life football, just look at how Barcelona play!
Playing with a team that features a quick, pacy winger(s) is incredibly useful. Teams like Manchester United who have great wingers like Cristiano Ronaldo on their team can create goals solely from utilizing his skill and speed. If you have a winger like this, it is not a bad idea to completely center your play around him, as if you get good at relying so much on your winger, you will realize how useful they can be by making quick runs down the sideline and playing a good cross in. It is a simple technique but considering how important individual pace is in FIFA 08, you'd be dumb not to take advantage of it.
Utilize Skill Moves
Skill moves are incredibly useful in all versions of FIFA, but players tend to ignore this. Whether it be because they are too hard to get used to or because they don't know just how useful they are, they are truly a hidden tool not to be ignored. Spending the time to get good at skill moves is well worth it, not only will you surprise your opponent by dribbling all over the pitch, you're missing out on a lot of scoring opportunities if you don't know how to use skill moves correctly. Something to consider is that if you want to play the all out skill moves game, look for a team with a lot of players with high dribbling stats.
Consider Manual Controls
Although it is very hard to break away from assisted settings and play with full-manual controls, it is well worth it in the end. This is the same concept as mentioned in the skill moves section above; there is a tight learning curve but you'll be upset you didn't make yourself learn it earlier! Imagine if your shots and passes were 100% controlled by you. Of course, you would have no one to blame when you missed, but once you are proficient in manual shooting, you can hit corners consistently and thus score more goals. See more on the manual vs. assisted controls debate here.
Utilize Set Pieces
This one is sort of a given, but set pieces are one of the most useful things in FIFA 08. Because of the importance of strength and physique of a player, more so than previous versions, all set pieces used as a pass are golden scoring opportunities. Free kicks taken as shots are something you should absolutely not ignore. Once you get good you will score maybe 3 out of every 4 attempts, and that's a pretty good ratio, enough to make your opponent think twice about fouling you.
Mix It Up
This isn't really a strategy, but something to consider when trying to play a certain way. Although it is nice to have strategies and ways of playing which work well for you, you may be forced to change it up every once in awhile. Don't always use your wingers, try to posses the ball a bit or immediately try to play your forwards in for a chance to finish. This is especially important if you're playing online because human beings notice patterns better than computers do and will act accordingly.