In my experience, relapse has occured for a number of reasons at various times. I've relapsed after a year clean, after 4 months, and numerous times after putting together a few days or weeks.
Most commonly for me was allowing my mind to trick me into beleiving that i'm not really an addict, that one drink or drug would be ok. It never works out like that, as the first one reignites my addictive thinking, behavior, craving...
To battle this i must remind myself daily that i am indeed an addict and do things to support my recovery. For me that includes going to meetings, talking to my sponsor and recovering friends, working steps, meditation, prayer, chanting, exercise, good nutrition, and the big one; avoiding at almost all costs the people, places, and things that were associated with my using.
A good tool that i use daily is the H.A.L.T. method of self-analysis. It's common that when i get too Hungry, Angry, Lonely, or Tired i can revert back to old behavior patterns and that can get your guard down and lead to relapse. the actions to combat these states of being are fairly obvious. If i'm hungry i eat something. If i'm angry i pause and take some deep breaths, maybe pray and/or chant for the person i'm angry at. If i'm lonely i call someone in recovery and/or go to a meeting. If i'm tired, i rest. It works!
I also mentioned a few pages back that i try to practice mindfulness throughout my day so that i'm aware of what's going on around and most importantly inside me. This keeps me in the present and on my toes with all the stuff that life throws at me. I facilitate this through daily meditation practice and the rest of the day i use a simple technique of pausing and taking three complete breaths before driving, getting on the computer or answering the phone at work, before interacting with customers or associates, etc... Whatever works for you...
Another simple technique i use when i find myself in daily life not being mindful is to say outloud to myself what it is i'm currently doing, rather than worrying, remembering, planning, fantasizing; all the behaviors that rob me of awareness of the present moment. For example, if i'm working on a job quote at work at find myself replaying a conflict with an employee earlier in the day over and over in my head, i will pause and say to myself "I AM working on a quote for _____." These I AM statements bring my attention back to the present moment and allow for mindfulness.
Good luck!
Everyone have a great day! It's day 28 for me!