Let's take a look at Ra, Ri, Ru, Re, Ro. The dreaded "r's" are probably the most challenging sounds for English speakers. Listen to the sound files over and over...
Write them out 10 times a day and soon you will get it. To aid memorization, make silly associations to go with them. You can use the above mnemonics or you can make your own!
アイスクリーム ____________ This
アメリカ ____________ This means ____________
ローソク_____________ This means candle (not from English)
キレイ _____________ This means pretty in Japanese (you may know this one already)
テレビ_____________ This means ______________ [NOTE: If you have studied