You just have to know where to aim from. It's especially easier on Domination game modes, where people cluster on capture points and you can safely launch nades from across the map if you know where to aim and where to stand. Like so:

http://www.youtube.com/watch?v=MSnQ9dh6Zwg

http://www.youtube.com/watch?v=xNFakcRv55k

http://www.youtube.com/watch?v=rDKpsfiU0mc

http://www.youtube.com/watch?v=DEx0Ce8nh6Q


Also here's a handy chart with tons of great info about guns and perks and such:

http://phatwater.com/MW2/