I have to agree with most of the other posts. Long courses that claim to train you to be a qualified flight attendant are not necessary. I've worked with and got to know hundreds of flight attendants over the last 7 years and I don't think I can recall any of them doing one of these long training courses. We all know airlines look for work experience, general experiences and personality types when employing.
Having said that, I did do one of those interview skill/preparation courses which was only for 1 day and I have to say the information I got from that was very, very helpful. Since doing that course (back in 1999!) I have flown for 2 airlines and been successful in 3 recruitment drives. The one day course was a great kick start and you get better each time you have an interview.