Okay, so first things first, no I'm not looking for tutorials here (unless it is a really good one that made a major impact on how you view modding). What I want here is any thoughts and insights that you have gained over the years, or how ever long you've been modding, that made modding easier for you. Or improved your mods greatly. Also, any techniques that you came across that helped you do things quicker.

So in general, I'm just looking for any general advice that you might have for a new modder. Just post the first thing that comes to mind that you do when making a mod, doesn't matter what kind of mod.

Test everything.
Test everything again.
When you're done testing, do more testing.

You never know how a player is going to play, so you need to anticipate for any variable they can come up with. Try really hard to break your mod, because chances are good someone will accidentally do it if you haven't tested everything.
It's very time consuming, but will save you from constantly needing to make updates/patches.

I agree with what Pluto said. Additionally, depending on the scope of the mod, some kind of checklist can also be helpful.