With learning Spanish I found that the easiest way to keep motivated was to keep it interesting. I didn't want to sit down with a dictionary learning verbs for hours, but doing it for five minutes now and again is not a great chore. The best thing for me was to use as many different learning methods as I could find but in small doses. I would find some resource on the net, use a course like the Michel Thomas Method reviewed yesterday, listen to a podcast, watch a little TV, read the newspaper, dip into a dictionary or simply chat to people. When it becomes tedious to learn is when you stop doing it, so don't let it become tedious.
All the methods I just mentioned work equally well for beginners or advanced learners, but obviously you need to adapt the difficulty of what you do to your level. As a beginner, browsing the newpaper headlines and matching them to stories you know about already from the English news can tell you a lot. As you get more advanced you will try to read more and more of the paper, but you don't have to stick with it beyond the point of boredom.