Here is a thing.

1. The final consonant 'ㅅ'('ㅆ')

1.1. 'ㅅ' + a vowel ☞ 'ㅅ'/s/ sound shifted to the vowel

ㅆ + o = x + ㅆ, 해써요/hae-sseo-yo/, 어 ☞ 써

1.2 'ㅅ' + a consonant ☞ 'ㅅ' sound makes the consonant stronger

ㅆ + ㄷ = /ㄷ/ + ㄸ, 핻따/hat-tta/, 다 ☞ 따

2. 'ㄹ' sound

2.1. words or syllables end with the final consonant 'ㄹ' ☞ sounds like /l/

글/geul/, 일요일/i-ryo-il/

2.2. 'ㄹ' + a vowel ☞ sounds like /r/

라디오 ☞ ㄹ+ㅏ(vowel) ☞/ra/

2.3. 'ㄹ' + 'ㄹ' ☞ sounds like /l/

걸리다 ☞ 'ㄹ' in 걸 + 'ㄹ' in 리 ☞ /geol li da/

You don't need to memorize all of the rules because when you pronounce each syllable exactly and make it fast enough, all are yours.^^'

\^o^/

I will explain ㅅ or ㅆ as in 받침.

1. When 받침 ㅅ or ㅆ meets ㄷ (잇다/있다), you pronounce it as [읻따]. You do NOT prounce it as [읻타], which is close to a T sound. When I say it really fast though, I say it close to [이따].

i.e. 웃다 (to smile) --> [욷따]

     씻다 (to wash) --> [씯따]

     있다 (to exist) --> [읻따]

     갔다 (went)    --> [갇따]

2. When 받침 ㅆ meets ㅇ (있어), then you pronounce it as [읻써], which is close to an S sound as in Sunny. Again, when I say it fast, I pronounce it as [이써].

i.e. 있어요 (there is) --> [읻써요]

     했어요 (did)       --> [핻써요]

This youtube video shows you how to prounce double consonants: http://www.youtube.com/watch?v=D9pR-mem_o4