Get a squat rack and a rolling bench! It unlocks many good lifts and is probably the sweet spot on the "exercises per unit of gym equipment" curve. A case for this approach to garage gyms is made in super-detail in [0].

[0] https://www.amazon.com/Starting-Strength-Basic-Barbell-Train...

Starting Strength is an excellent book for getting started with lifting.

https://www.amazon.com/Starting-Strength-Basic-Barbell-Train...

Doing these two things has worked extremely well for me:

1) Doing deadlifts. This is a strength-training exercise: https://www.youtube.com/watch?v=4AObAU-EcYE

2) Seeing a good chiropractor about twice a month. (You might need to shop around - I can elaborate on how to select a quality one if people want)

You asked about an app. I actually corrected my earlier posture and lower-back problems using the above and not any kind of app to remind myself to get up and move around; the above helped even though I routinely sat 1-2 hours at a time, in a deep coding trance, without moving around. Recently, though, I started using BreakTime (a macOS app) to remind me to look away from the screen, mainly because I believe it's healthier for my eyes. This is a $5 app I've been quite happy with.

All the above is in the anecdotal "works for me" category. Good luck in your effort to find what works best for you!

Edit: The video link above gives a rough sense of the exercise, but the fine details matter greatly, so if you decide to do it I highly recommend studying the relevant chapter of this book: https://www.amazon.com/dp/0982522738/

Edit 2: More about chiropractors: Few are MDs; the ones I've worked with all went through a 4-year, post-bachelors program. In my experience, chiropractors fit on a spectrum, going from ambulance-chasing quacks to gifted, remarkably effective healers. You want the latter. Only way to find out is to pay for about 3 visits; if you and your body are consistently feeling better physically and emotionally by then, you've found a good one. If not, stop, and go find another. So far, the best I've found lean towards alternative medicine (which was really surprising at first... I'm trained as a physicist, and was REALLY skeptical.) Can't say that's a general rule though. Again, all this is my experience; better to get your own experience and evaluate based on that, instead of blindly deciding based on something you read online.

I've really enjoyed Mark Rippetoe's writing about the difference between Exercise and Training.

From the gloss for Practical Programming for Strength Training [1]: "Exercise is physical activity for its own sake, a workout done for the effect it produces today, during the workout or right after you're through. Training is physical activity done with a longer-term goal in mind, the constituent workouts of which are specifically designed to produce that goal."

He emphasizes that a realistic means of progression is one of the most important aspects of any training program.

Practical Programming and Starting Strength [2] (which is a better introduction to these ideas for novices) are together the best analytical discussion of physical training that I've seen.

[1] http://www.amazon.com/Practical-Programming-Strength-Trainin...

[2] http://www.amazon.com/Starting-Strength-3rd-Mark-Rippetoe/dp...

Here's a link to Starting Strength[1]. I highly recommend it for people looking to bulk, tone, and put on some real muscle. It did wonders for me and I'm sure it will for you too. I also recommend doing these powerlifting exercises (such as squats, bench, and deadlift) with a focus on FORM first instead of weight. It'll prevent you from getting injured and payoff when you start lifting real heavy weight. I highly recommend these videos to teach you powerlifting form[2][3][4]. Also, the nice people on r/fitness over at reddit won't mind giving you a form check if you post a video of you lifting. Seriously, start now. It'll change your life.

[1] http://www.amazon.com/gp/aw/d/0982522738?pc_redir=1400896569...

[2] http://youtube.com/watch?v=2ME8gEN54Ao&feature=kp

[3] http://youtube.com/watch?v=EHx1gYTA-Rw

[4] https://www.youtube.com/watch?v=xS2wLZSdeDg

2-3 times a week, strength training, following "Starting Strength":

http://www.amazon.com/Starting-Strength-3rd-Mark-Rippetoe/dp...

Just a few sets of bench presses, shoulder presses, squats and deadlifts, a couple times a week, seems to do wonders for both appearance (and posture) and mood (hugely). Because it's so efficient, and actually feels good, going to the gym is something I now actually look forward to.

another dark secret: form is not the #1 priority. youtube is full of videos of people doing crossfit with horrific form and the instructors seem more focused on getting folks to "do" the exercise rather than "do it right".

it seems like crossfit doesn't quite appreciate how easy it is to destroy a knee, or your back, from improper deadlift or squat form. done properly, they are fantastic exercises. improperly, they are dangerous.

http://www.amazon.com/Starting-Strength-3rd-Mark-Rippetoe/dp...

For anyone interested... if you want good posture, if you want to really be in shape, there are a million fads out there, but the best book by far is "Starting Strength" [1]. (It's also one of the best-selling on Amazon.)

It essentially focuses on just the squat, deadlift, press, bench press, and (later) power clean, devoting around forty pages to each, and explains why you really don't need much else. They're quite difficult to get right, but the incredibly in-depth explanations will especially appeal to programmers who like understanding how things work.

I say this just because the book completely changed the way I approach the gym, and it mirrors what the article author says about the exercises he used.

[1] http://www.amazon.com/Starting-Strength-3rd-Mark-Rippetoe/dp...

If you are interested in free weights, Starting Strength is the best book out there. In free weights you have to be extremely careful about your form otherwise you could seriously injure yourself. This book goes deep in the human anatomy and mechanics to teach you how to approach free weights.

http://www.amazon.com/Starting-Strength-3rd-Mark-Rippetoe/dp...

I plan to switch back to weight training around March and give that a try for a couple of months to see if I can find a way to make that work better.

Try Starting Strength ( http://www.amazon.com/Starting-Strength-3rd-Mark-Rippetoe/dp... ). Most people (read: me) dick around in the gym with bicep curls and nautilus machines, which is a mistake.

I got basically no results while doing that, and my improvements have been dramatic since I started SS.