First you need to define more clearly what the problems are - individual sounds or rhythm and intonation - and then deal with one at a time. As they're Spanish speakers they'll undoubtedly have problems with individual sounds, and I'd start with that. You can find out which sounds cause problems
here
Deal with one sound at a time. A good technique for this is minimal pair work. A minimal pair is a pair of words which differ in one sound only - for example
chip/ship, chop/shop, chin/shin, cherry/sherry. Choose words you can easily get pictures of. Show the students the pictures and teach the words. Have written labels so the students can concentrate on pronunciation without having to worry about remembering vocabulary at the same time. Then put them in two columns -
ch words and
sh words. Make sure the students understand that all the
ch words are on one side and the
sh words are on the other. Make sure the students can hear the difference (don't take this for granted) by first saying one of each pair at random. The students point to the correct picture. When they're doing this OK, it's their turn. They say one of the words at random and you point to the picture that you think they mean.
Once they can say the words individually, try putting them in simple sentences for repetition.
There are lots more techniques you can use for teaching individual sounds, and still more for rhythm and intonation. The following websites may be useful, or just type EFL teaching pronunciation into Google.
www.teachingenglish.org.uk/try/prontry/pron_tips.shtml http://www.eslflow.com/pronunciationlessonplans.html http://esl.about.com/od/pronunciationlessonplans/