Hopefully by now you're all at least a couple weeks deep into a lifting regiment, and working towards that 15% increase. Now I want to address an equally if not more crucial aspect of fitness, diet.
Now, by diet, I don't mean a specific program, but how you eat. It's important your body gets the right amounts of the right types of food, or your time at the gym is next to useless.
I'm going to go out on a limb and say most of your diets are off in some way. Most of you eat too much, some of you eat too little, and only a few of you eat the right amount. I'm going to break this down into losing, gaining, and managing one's weight, but the basic idea is to manage your caloric intake with your caloric expenditure.
Losing Weight: This is the camp most of us are (or used to be) in, myself included. The basics of this are to make sure you're at a caloric deficit, meaning you burn more energy (calories) than you take in. This means eating less and/or exercising more. Increasing your strength or muscle mass is tougher during weight loss, but can be done using a /r/keto diet and the resources over at /r/leangains. A lot of guys like cardio for weight loss, I don't, as I find it inefficient. I prefer to eat a lot of protein and lift heavy (as you put on muscle your caloric expenditure will go up). At a basic level just pick a diet plan which restricts your calories and stick to it (that's the big secret).
Gaining weight: You lucky little bastards. Ok, that's probably unfair of me. But the good news is, you get to eat mostly whatever and whenever you want. The bad news is, you have to eat 500 calories a day after that. You want a caloric surplus (in>out). You also want to lift, lift, Lift. Otherwise you run the risk of gaining fat rather than muscle. You most likely will gain some body fat, but that can be taken care of. (an you might actually need it.) /r/gainit are good resources for bulking.
Maintaining Weight: Hopefully, with time and effort, you all will at some point be in this category. Here, you want a caloric equilibrium (in=out). Here, it's simply a matter of maintaining your lifestyle. Stay active, eat healthy. Also, don't freak about gaining 5-10 lbs over the winter, it happens to everyone, just go back to the losing weight section and get rid of it. /r/fitness is a good general fitness resource.
Note: All on sub posts should be pertinent to the subject of the monthly challenge. Questions should be posted in the most recent AskBaM thread (typically there will be a new one bi-weekly). Posts not following this rule will be redirected/deleted.