What does it take to succeed in this dive?
Like studying any foreign language, to succeed with Shanghainese, you will need to spend a lot of time reading, writing, speaking, and listening. These four skills together will help you to master Shanghainese. 
The hardest part about learning Shanghainese is the first three months, as you grow accustomed to the characters, and the tones.  This is not unique to Shanghainese but to all of the Chinese languages/dialects.  
Once you get over that hump, it then becomes merely a matter of constantly adding to your vocabulary, as the grammar is relatively simple. For example, unlike many western languages, there are no verb conjugations in Chinese. Ahhhh, how nice. 
You will have to find good study materials, good teachers, and good practice partners. Most importantly, you will have to find the discipline to study and practice and improve regularly. If you don't study regularly, you won't make much progress, or worse, you'll make lots of progress and then forget what you've learned! 
The secret to fluency (since we know you want to know) is quite simple: master reading, master writing, master speaking, master listening, improve your pronunciation and improve your vocabulary. The issue is that most people don't actually do any of these tasks well. You see, most people only focus on one or two of these skills, or they focus on all six, but at a very cursory level. If you wish to become fluent, however, you need to focus on all six, all the time, and spend enough time with each so that you see regular progress. 
Bottom line, if you want to become fluent, do this: 

  1. Read 20 minutes a day in Chinese, and from varying sources 

  2. Listen to Shanghainese TV and radio every day 

  3. Write a new essay or short story every week and get it corrected 

  4. Speak weekly with others, and ideally, those who can correct your mistakes 

  5. Make sure your pronunciation is as spot-on as can be 

  6. Improve your vocabulary constantly 

Do these things regularly, and within 4 - 7 years, you can almost certainly become fluent in Shanghainese. The question is: will you actually do these things? Ahh, that is the question. We hope you will!