One thing that's very important in Japanese pronunciation is to keep all mora the same length. This is difficult for most English speakers, because English emphasizes syllables by lengthening them to about 1.5 times the length of the unaccented syllables. In Japanese, one kana equals one morae. This is different from syllables, which don't matter too much in Japanese. Take the word 最近 (saikin) for example. This word has two syllables: sai and kin, but it has four mora: sa, i, ki, and n. In order to get the rhythm right, get a beat going in your head and place all of the mora on one of them, trying to keep them all equal. Then work on speeding them up, and making them sound more natural (getting the accent right also helps, but one thing at a time). I actually do this to make my Japanese sound more "native," and I find that it helps a lot. If you have a metronome I believe that would tremendously help you as well.