My favorite question!
1) When ㄹ occurs in the beginning of a word, the standard pronunciation is supposed to be L. For example, the Korean word for "Russia" = 러시아 (written 로씨야 in North Korea) is pronounced with an L-sound by newscasters but many speakers (myself included) generally pronounce it with a lightly trilled R.
2) If ㄹ occurs in the middle of a word and the syllable before has a vowel -- for example, the Korean word for "wind" or 바람, it should always be the lightly trilled R sound, never the L sound.
3) If ㄹ doubles (or ㄹㄹ), then it should always be the L sound -- for example, 말리다 ("to dry" or "to break a fight", depending on the situation) -- should always be pronounced with an L sound.
If you have any other questions, send me some words with ㄹ, and I'd be more than happy to teach you which sound you're supposed to use.
:)