Rep is a hard thing to explain. Rep is given out by the other members of the forum, when they like your post or finds them helpful or just plain funny. There are many other reasons members give out rep, those listed are just some of the main ones. The more helpful and insightful your comments are, especially if the original poster is asking for advice or help, will generally give you a better chance of having that member give you some rep.
So techinically, there really is no solid formula for how to get rep, but I will give you some ideas.
-Post up pics of your models. Members like to see models, well painted models or cool conversion ideas and most members are pretty generous when they see cool things. This is where I give out most of my rep and where I received the most as well.
-Give really good advice. If someone is asking how to use an army, going over the norm of just saying get these units. Provide different tactics, how to use units together etc. If they are asking about painting, provide step by step detailed instructions. Providing a detailed answer is always appreciated and is usually rewarded.
-Create a tutorial. Got a really good paint idea to save time, know how to make awesome bases. Post up the steps, with pictures is even better. If folks like it, the rep will flow in.
-Become a supporter. That will usually get you a nice amount of rep, right off the bat.
I used to roll the dice, Feel the fear in my enemy's eyes
I'm afraid there's no authoritative source for thinking you're an asshole