I learned katakana a row a day. I don't like the all at once method, since you will have a harder time getting fluent in reading it. However, thats different for every person.
The one site that has helped me was:
Real Kana - Practice Hiragana and Katakana Just whenever I had a spare minute I practiced my rows.
But writing the characters you know a couple of times a day (and eventually just once a day or w/e) helps alot.
Also, if you have trouble seperating kana from eachother just take a few spare minutes and observe them both very very good and compare them to eachother so you know exactly what the differences are. I never had any problems at all seperating ン from ソ.