I'm pretty new to Gunpla, too. But the single best thing I learned is that Gunpla is Freedom! or, in other words, there are no rules. So you're free to explore and experiment and develop the way you think is best. That said, the best things I've learned have been here and on youtube.

One thing I learned from my dad, though is how to use buffing pads properly. If you use them right, it'll solve the scratched up look you get after sanding your nubs. You can get these buffing pads at any hobby store and they usually come as small squares in an increasingly fine grit. I originally thought they were sanding sponges... they are not sanding sponges. How you return the shine is you take the lowest numbered pad, first, and rub down the mark and proceed to do the same with the next highest number all the way to the highest number. It's pretty nifty to watch a piece of plastic you've marred slowly roll back to it's shine in the palm of your hand.

One other thing I like to do, which I learned here, is to use plastic cement to erase unnatural seam lines. Especially in my weapons. I hate those weird seam lines. What the plastic cement is doing is melting the plastic so when you bring two of the pieces together, you're then making one piece.There's a tutorial on how to do it in the repository here.

Based on this principal, which I figured out on my own... although I'm not the only one who's thought of it, was to use plastic cement and my runners to repair mistakes or blemishes on my model. I'm still trying to perfect this technique, and newer models almost never call for this technique, so I'm not sure I can properly explain how to do it. I'd also point out that, for the very reason that it works, it could also really mess up your model. So meybe experiment on another part of the runner to really get the hang of spreading the plastic around before trying it on your model.