Running and fitness. kleinbl00 (I swear that's my last shout-out to you tonight) can attest to this. He pointed out an app to me called "myfitnesspal" that I was rocking for a good few weeks in a row and I've completely fallen off the wagon. I was running etc and now I'm back to feeling tired, fat and unmotivated to do anything active. I go in waves where I'll be really, really fit and active and then all of a sudden, I stop. Granted, I don't have a ton of time during the day to run since my son was born, but still I could make it more of a priority than I have of late. I'm definitely "streaky."
If I may suggest something that helped me: Find a program, do a program, find a new program. http://www.bodybuilding.com/fun/find-a-plan.html Nobody can stay motivated doing the same thing forever. I recently ended an 8-week training program targeting strength and mass, then froze my membership and started running and am working my way into a steady calisthenic program (bodyweight exercises, easily done outdoor with a pull-up bar and other jungle gym stuff). Diversify and keep your original goal ever everything-- for me, its meeting West Point physical exam requirements, and having confidence in my strength. How I do it is therefore flexible. Those apps ain't gonna work for you, they're for you to look back on and be proud that you worked, that's it. If you mainly run and are losing motivation, fighting time constraints etc. but don't want to find something different, change up your run. As in, change up how you're running, short distances or interval training etc.
I love bodyweight exercises and highly encourage people to pick them up. Bodyweight exercises have just about the lowest barrier to entry in terms of working out - you don't need any equipment, you don't need to leave your house, you barely have to change out of your business clothes if you work during the day. Heck you can sneak some in at work if you choose some subtle ones. For those of you that use things with apps, there is one called "You Are Your Own Gym" by Mark Lauren. It does cost money (I may have gotten it for free on a special one day, can't remember) but it provides an extensive library of bodyweight exercises (GUYS DID YOU KNOW THE CRABWALK IS A BODYWEIGHT EXERCISE?) as well as programs and steps to help you achieve certain milestones - for instance, if you want to be able to complete a pull-up. It has videos as well as descriptions of the exercises. I'm also big on incorporating fitness into daily life. I walk the stairs at work when I can, sometimes instead of using a bathroom on my floor I walk up or down a flight and then use the bathroom for example.
Important thing to note is how hard it can get. If you've been hitting the gym for a while sure you might knock out some push-ups or a good plank, but how about a set of muscle ups? A human flag? There's a lot of room for progress. Also your body burns fat way better and the muscle it builds, on most guys at least, is insane.
I have myfitnesspal, but I never use it, even though daily fitness is important to me, as is calorie counting. It's a good app, and using it wouldn't hurt me, probably would even help me. But those days that I overeat, it's hard enough to know I've overeaten; I really, really don't like putting it down in numbers. What I do personally is I have a set of daily goals and I use the app "Don't Break The Chain." DBTC is really simple, and you can also use it as a website - one chain is free, if you want more than one chain on the app you have to pay (I have 7 chains). Anyway, every day that you achieve your goal, you just tap on (or click) that day, and an X appears. I like it because most of the time I'm interested simply in knowing, "Am I really making progress towards my goals?" I like to quantify my progress; I like to know how many days I've done something vs. how many I haven't. I also find that running multiple chains reduces the feeling of "failure" if I do not achieve at one. My goal is usually to hit 3-4/day, any more is gravy. I consistently fail at "Didnt Go Out." 3 goals are writing-related and 3 goals are fitness-related...wonder what I prioritize :) 
