What is the most common way to say 'indecisive' in Mandarin?
I find two meanings of "indecisive" in the dictionary.


  
  To hover between different choices and can't make a decision in a short while. 
  Not specified, undefined.
  


I think you are asking a word for the first meaning, and then the words are:


  犹豫不决 
  优柔寡断 
  没主见*
  徘徊不定 
  犹豫


There are some other words related to this meaning, in different contexts. Besides, I think you are talking about "磨蹭" by "磨(it may mean the same in some dialects)" in your comment.  "磨蹭“ is different. It's used to say someone intends to do something in a very slow process, thought he is able to do better or faster. I know the English words "to loaf" and "to linger" for "磨蹭".

Edit: To tell the difference.

You can use "优柔寡断" and  “没主见" as an adjective to modify a man or his character, while other words can't be used in this case. You can use "徘徊不定","犹豫","踌躇"(also "优柔寡断" and  “没主见"),etc. when talking about something, and you need to remember some patterns to express such a meaning. Example sentences to help me explain:


  他的性格优柔寡断. He has an indecisive character.
  他是个没主见的人。 He is indecisive.
  他在提名谁做总经理的事上徘徊不定=他在提名谁做总经理的事上犹豫不决。 He is indecisive to nominate who as general mangager.
  我犹豫要不要接受他的邀请。I am indecisive to accept his invitation or not.
  别犹豫了！Don't be indecisive! (make your decision as soon as possible!)



没主见 more precisely means "not have one's own opinion, one's opinion is easily to be swayed by someone else.", while other words don't express the sense of "easily to be swayed".