I usually get most of them just playing the game. The rest are usually in other game modes that are put there I think to get people to try out all aspects of a game. I don't really waste my time on achievements all that much though, I mean its not the reason I buy games and spend time playing them. As it is I spend most of my time working and I don't think its worth it to use the precious little game time I have to hunt for achievements.
It is nice getting that achievement icon pop up though, I won't lie to you.
