I had great help from the resources on MR. Haven't used them for a while but from recollection:
I like how it's split into different classes of words, doing one at a time.
Any tests are very useful so that you can see your progress and find out which areas need more attention.

Although my Russian is truly terrible - I managed to travel through some CIS countries solo - almost entirely without speaking English. I like the language, the culture and the people a lot. What started out as a minor hobby, simply learning a skill that might be useful for work, has turned into a real passion.
Now I will take a real course.