It's actually pronounced two ways - a tap and a flap - depending on the word, but your tongue should end up somewhere around the back edge of your alveolar ridge (the bony part not far above your teeth). An English L makes contact with the front side of it. There are some more details in this thread
http://forum.koohii.com/viewtopic.php?id=7770 There's a much longer thread on the topic on the forum somewhere as well. After you understand the technique, try going to
http://www.csse.monash.edu.au/~jwb/cgi- ... dic.cgi?1C and repeating some ラ行 words until you can train your tongue to do it correctly. It's also worth practising your vowels, since pretty much all your pronunciation will sound off if they are.