I have the same problem as most Asian Spanish learners with the Spanish single 'r' and 'l'. We usually mispronounce 'ala' as 'ara', and bra, bla. bre.ble .... etc are difficult for me to speak them accurately. So how should I pronounce single 'r' and 'l' correctly?
I think the basical thing is that, when you have your tongue up to pronounce "r" you have to make it vibrate, and to pronounce the "l" you don't (you better push your tongue against your palate) .
It actually depends on where the r is located in the word:
At the beginning after «l», «n», «s» or prefix «sub-» is pronounced as /r/
In other positions is pronounced as /ɾ/
l is always pronounced as /l/
You can check this pronunciation page to listen some samples.