I switch between 3-4 formations depending on the match;
A wide and a narrow variant of 4-2-3-1:
Wide:
GK
RB CB CB LB
CM CM
RW --- AM --- LW
ST
Narrow:
GK
RB CB CB LB
CM CM
AM AM AM
ST
The narrow one is my main formation, I switch it to the wide one depending on what formation the opponent plays - if they have good defensive midfielders then I play the wide one to try to avoid them.
Against the tougher teams I switch to a 4-1-2-2-1 type thing, as follows:
GK
RB CB CB LB
DM
CM CM
RW ----------- LW
ST
Finally, when playing as Arsenal, I adopted a different formation which I had success with - its great if you have the players for it and want to play Arsenal/Barca style football.
GK
CB CB
RWB --- DM --- LWB
CM CM
AM
RWF --------- LWF
(WF as in Wing Forward - a nice fast striker to play out wide).
The key with this formation is you need a very good AM who will attack the box in the absence of a Centre Forward, and your DM needs to be the opposite, pulling back to join the CBs when the Wingbacks attack.