What is the difference between どうもありがとう and ありがとう??
Sometimes I hear somebody says "どうもありがとう"
Is adding "どうも" make it more polite??

Question

What is the difference between どうも ありがとう and ありがとう??
Sometimes I hear somebody says "どうもありがとう" 
Is adding "どうも" make it more polite??

Answer

Yes, adding どうも makes a sentence more polite.
どうも is convenient word. 
You can say, どうも すみません(I'm sorry), too.