Fortunately for them, there will a new crop of Democrats to run against, so the field will be clearer. The necessity of opposing Obama on all fronts at all times won't be necessary any more. That distortion caused by the right did contribute in ruining them.
The first thing that they need to do is to actually adopt the real problems of the day. All too much, the party has been concerned with boogey-man problems that energize their own base, but don't actually pose substantial problems to the electorate. That will be tough. Unfortunately, the party leaders tends to believe their own propaganda machine. The ends up creating a primary that creates candidates ill suited to winning the center. But if the party does persist in wrestling with the wide problems of the day,
Put down the culture war. The effort placed on cultural issues is just not giving enough return. On the other hand, do adopt "everybody" cultural values. If you are going to sell your brand, sell it for everyone. "I want good jobs for all Americans. I want safe neighborhoods for all Americans. I want safe food and water for all Americans. I want a secure world for all Americans. These are important things and worth paying for, so I will stand up to keep these things healthy and well funded, while watching over them, to be sure that our good intentions don't produce waste and abuse."
Stop the war on the government. You will regain power eventually, and if the people hate the government enough, then they will hate you as well. In fact, the more that you attack government, the more that people won't like congress. You get the idea. Stop the stupid idea that "government is the problem." You've won that battle with Reagan.
Know what you want to achieve. Opposition won't win the next election. You do need plans, but not false plans that don't serve their intended function. To win respect, you must be speakers of the hard truth, and hold that hard truth up against the president. The rank and file may not appreciate that truth, but in time they will come around.
Educate your base. Government is hard. Help explain how and why it's hard. That will require them understanding how the government works. That's good. They need to know what can be done easily, and what takes huge effort.
Finally, if member of your party makes your brand look bad, then your brand looks bad. Don't tolerate a few people making the majority look bad.