I agree with superkojiman's comments. His recommendations were helpful to me when I was preparing for my exam retakes.
Understand the labs and if you don't understand something research it further. I went through several buffer overflow tutorials before I got it. Like superkojiman said enumeration is key. Nmap is not always enough. On webservers, I would run nikto, dirbuster, and httprint. This will help you find vulnerabilities and httprint is a good crosscheck to verify you have the correct webserver and version.
It took me a while to think like a hacker, once I did it got easier to root servers in the lab. That comes from practice in the lab and the understanding the exercises in the lab manual and the videos, it will help you learn to think that way.