"I've been slacking off at exercising recently, but I decided to make a concerted effort to be more active."
"I'm making a concerted effort to be nicer to my roommate."
"I'm behind in my classes, but I'm making a concerted effort to catch up."
Concerted effort is a popular phrase to misuse these days. Concerted means "involving the joint activity of two or more", so the idea of a single person making a concerted effort doesn't really make sense.
- concerted: coordinated, combined, joint (opposite of separate, uncoordinated)
- concentrated: intense (opposite of mild, moderate)
The next time you're about to say concerted, think about whether concentrated might be a better choice.
(Although it's widely regarded as incorrect today, my guess is that in 20 or 30 years this use will become accepted, much like "I could care less" is becoming today.)