Playpal - it really depends on the city. In London, a lot of clubs have a door policy but seem to ignore it. In Dublin, they seemed not to give a damn. In Paris, you're hosed if you're a foreigner and not dressed to the nines. Also depends on the type of place - the glitzier and bigger a name the place has, the stricter the door policy will be. But then, places like that are too expensive and chichi anyway
In general, expect to run into a no trainers/no athletic clothes dress code. You can't go wrong anywhere w/ dark pants and a button-down shirt of some sort, and shoes that don't obviously look like sneakers. Except freakin' Paris.