Help! Remediation of Integer Operations

I am currently in my 3rd year of teaching 8th grade mathematics after teaching four years at the high school level.  During the first year of teaching 8th grade, I came to the realization that the majority of my students did not understand the concept of integers and operations with them.  The truly sad part of this is that almost every 8th grade concept that I teach depends on that understanding. Solving equations and inequalities, finding a rate of change from a table of values, rules of exponents, finding probabilities of compound events, and solving systems of equations are some of the concepts that are directly affected by the students' lack of understanding of integers.  I have no doubt that their 7th grade teachers taught the concept thoroughly.  I have been reflecting on this for the last three years since it directly affects my students' success in 8th grade.  I find that I devote a great amount of time reteaching integers so that my students can comprehend the concepts that are in the 8th grade standards.  Why do students have such a hard time with the this?  Is it because of its abstract nature?  Are their 13 year old minds not developed enough yet?    In an effort to help them, I give concrete models such as movement on the number line, colored chips, etc instead of just repeating rules that they memorize but do not understand.  Recently, I found these resources ( http://ow.ly/dUyXD and http://ow.ly/dUz5a ) that I feel would be beneficial.  I would really like to hear from other teachers who teach integer operations and have found something that works for their kids. What works for your students?  Do you have a different approach to this concept?  Any comments or suggestions would be greatly appreciated and shared!