Use barriers to your advantage by making goal oriented behaviors convenient and goal sabotaging behaviors inconvenient. Here's how.
How many times have you packed your workout clothes only to return home having never opened your gym bag?
How many times have you gone to work with great intentions to eat well only to grab a less healthy option because it was more convenient?
We often beat ourselves up when we have days like those, but if we step back we can often notice the reason it was destined to play out that way. Usually it's because of invisible barriers that our best “intentions” go unfulfilled. We need to recognize barriers and then use them to our advantage.
A barrier can be anything that makes it harder to meet your goal. Say, your goal is to run 4 times per week, but you're typically missing that fourth run. Seems like something always “pops up” and causes you to skip it. The barrier could be the time of day, that it's simply not convenient enough, or you that you can't seem to hold yourself accountable.
What if you could make it totally convenient to eat really healthy stuff while making it really inconvenient to eat unhealthy?
What if you could make strength training, running, or just playing really convenient while you had to go out of your way to avoid it?
Think you'd be better off?
Avoid The Lone Wolf Mentality
My number one suggestion is always to join a group or buddy up with someone else who's got a similar goal. For some reason, most of us have a harder time keeping a commitment with ourselves than with someone else so let's use that to our advantage.
Whether this looks like a training partner, a group, 
or even an online community or accountability buddy, this can be a game-changer. Do not underestimate the power of social support! Start by enrolling those closest to you so that they know what you're up to and can help you.
--->Hint: perhaps you slack off when it comes to speed work? Join my training club, Philly Surge Running and get faster! It's WAY easier if everyone else is doing it, right?
Reduce Your Reliance On Willpower
Willpower is something that you have in limited supply. Draw on it frequently enough and you've got none left.
On the commute home from work you wrestle with yourself about running after work and sometimes you'll win and go for a run, but half the time you'll “lose” and decide it's more important to clean the house or make dinner because you're hungry.
Folks, no one makes good decisions at the end of the day. Set up a barrier so that you don't have to be faced with such difficulties. Train early. Join a group. Meet someone along your route. Run from work. Track your training in a training log (these can be deceptively motivating – I use Strava).
How about a nutrition example? Walk by the cabinet that has the box of cookies frequently enough and eventually you will have one (at least!).
The “law of availability” (<--- read this! Its all about kitchen makeovers!) states that if it is available, then you will eat it. So what kind of food is available in your house right now? Make it easy to eat healthy by keeping junk out of the house. Make it so you must go out of your way to choose unhealthy foods.
Oh and watch out for justifications like this one:
“But it's for my kids.”
Really? Great habits start with the parents. Set a good example, k?
You get the point – set yourself up so that you don't have to make these seemingly little decisions everyday. Realize it's easier to rely on willpower early in the day, but really tough later. Discover and identify what barriers are in your way and come up with a way to reduce the height of them. Set up “helpful barriers” like group sessions that help you stay on track.
Other than the examples I've given, are there any barriers you can identify that might be working against you? What would you do about that?