I don't know anything about Rosettastone but I have done a bit of language learning over the years:
Firstly, a dictionary - if you are serious about learning you will need one. I find charity book shops a very good source of cheap but good condition dictionaries (try Oxfam in Clifton as I see that you're from Bristol). I can never have enough of them - it's useful to have a several small ones to leave in the office or around the house - they do wear out with intensive use
Other courses - for a beginner the Michel Thomas courses are good - very portable if you copy the CDs to an mp3 player.
There is no substitute for direct human interaction - evening classes, one to one sesssions are very effective. Also bear in mind that for most adult learners the most effective way to remember things is through writing them down.
Another useful tip is to try and listen to as many native pseakers as you can - internet radio stations are very useful for this.