Disclosure: I actually never used reddit for fitness advice, but it does look nice enough that I wish it had been around earlier! I used to be unfit, but I got myself sorted out before reddit really came along. To find good advice I had to read a lot of trash along with the good and evaluate everything critically. If you throw enough ideas at the wall eventually some stick. So in that spirit, here are some of the revelations that helped me the most.
-Gyms suck. The ideal gym member buys a life-time membership and then shows up twice. As such, many gyms aren't really set up to help you form good habits. What do even good gyms really offer that you can't have at home though? Exercise machines let you isolate specific muscles and avoid injuries that can come from bad form when using free-weights, but free-weights really are better if you learn proper form. Free-weights force you to develop stabilizing muscles and generally won't let you neglect muscles that are needed for real-world movements the way some machines do. If you train with free weights you're less likely to injure yourself playing sports. When you're twenty-something injuries aren't a big thing because you heal so fast, but after thirty they suck and you really need to avoid them! Free weights are also really cheap. In fact, you can find people giving them away for free on a regular basis.
-Rowing machines rock. Even if you live someplace that's always warm (I don't) you should try to get a little variety in your cardio workouts. Owning a good cardio machine can really help with that. Unfortunately, most home exercise machines are built for the same people who buy lifetime gym memberships and then never use them. i.e. They're not built to last very long if you actually use them. I wore out a few cheaper machines and then decided to find something that would last. Elliptical's can give you a pretty good workout, but good elliptical's are horrendously expensive. You can get a fantastic rowing machine for less than $1000, and they can really kick your ass. (I generally don't like giving away free advertising, but concept2 really is worth taking a look at here.) Why aren't rowing machines more popular? They make you look ridiculous and bad form can lead to injuries. Learn to row properly!
-Listen to your body. Our bodies do a great job of telling us when they're hungry, what they're hungry for, and how much food is enough, albeit that last one with an annoying amount of lag! A hugely beneficial healthy habit to form is not to eat when you're not hungry. We get psychological cravings for food that we simply don't need to eat. Learn to subvert those. Other times we crave something specific, like fat or salt, and then eat something "healthier" hoping that it will satisfy us only to wind up eating what we were craving in the end anyways. End result: we just ate more than we would have if we'd have just eaten what our body wanted in the first place! Forcing ourselves to eat too much healthy food is unhealthy, as bizarre at that may sound. Our bodies do tell us when we've eaten too much, but only well after we've done so. It's okay to stop eating while you're still hungry. In fact, you really should stop eating while you're still hungry. It takes time for us to feel sated no matter how much food we shovel in. You may be surprised at how little it takes to feel sated versus how much it takes to feel bloated! Give yourself time to feel sated and, if you still feel hungry, eat a little more. That way you're eating to not feel hungry instead of eating to feel full. There's a big difference!
-The most important workout is the one you least want to do. You slept poorly last night. You have a cold. You had a rough day at work. You have every excuse to take it easy, but if you get that workout in, every other workout you do will be that much easier to motivate yourself to do. If you start cutting yourself some slack pretty soon you'll be skipping workouts because a dog looked at you funny.
-Do the math when reading food labels. 1 serving is not necessarily what you'll eat. e.g. Potato chips frequently list the nutritional data for a serving size of "10-12 chips". That is not how normal humans eat potato chips.