back and forth: in a backward and forward motion Examples of BACK AND FORTH She and I went back and forth, continually trading ideas, but we never reached an agreement. My job has me going back and forth between Atlanta and New York right now.

to stick to: to adhere to (a promise), to follow or obey (a set of rules, procedures, etc.) Examples of TO STICK TO All organizations expect their employees to stick to established work rules and procedures. If you try hard to stick to your principles, then you’ll be able to live up to them.

to build up: to increase slowly, to make stronger gradually Examples of TO BUILD UP They certainly have built up a vibrant user community in a short period of time. These deficits have been built up over the past 10 years and nobody said a peep.  

clear-cut: clearly stated, definite, apparent Examples of CLEAR-CUT Zettel recently visited Michigan and said that school is his clear-cut favorite. Such success made him a clear-cut selection but tells a thin slice of the story.

to set out: to start traveling toward a place; to arrange or display neatly Examples of TO SET OUT We set out for the top of the mountain at dawn. Unfortunately, as we set off, it started to snow heavily, so we decided to head out again later. The children tried to set out the…

to drop out of: to stop attending; to withdraw fromThis idiom can be made into the noun form dropout. Examples of TO DROP OUT OF Your article notes that many students drop out of school because they are bored. They were also less likely to drop out of university or fail to obtain a degree.