Goddamn I love loops and poles
If you don't know what loops and poles are, google for 'loops and poles blenderartists thread'. You are cutting your own arm off if you model without this awesome bit of education.
Loop flow isn't just an organic modelling consideration, I was modelling a gun recently and I had to pay attention to the loop flow of the mesh every step of the way. When baking down high resolution meshes it's vital that you avoid any smoothing artefacts, and knowing about the flow of your mesh and being able to use poles to target and direct the flow is a paramount. If you need a smooth crease in your mesh ie for a wrinkle in a face or a fold in the handgrip of a gun, you put a loop there.
Loops are one of those concepts that just miraculously work every time and make everything better. They're like the midas touch for your meshes. It's like a missingno cheat to get rare candies for your modelling skills. I cannot count the amount of times I have had a particularly problematic area on a mesh and one well placed loop has fixed it. Or the amount of times I have put in a fresh loop only to have the other end of the loop fix up a spot that I wasn't happy with.
Poles make my pole pole up
If you think that sounds gay you need to start using poles. Every mesh needs them. Even a cube. Actually, a cube is all poles. By knowing how to place poles and how to move them around you have the entire world of modelling at your hands. A loop needs a pole to tell it what to do, and if you ever have a bad looking pole that means your mesh is about to fall apart. When I first started learning poles I thought I could get away with a 6 pole here or there, and my meshes fell apart too. Then when I went in to fix it up I found every time that it was the 6 poles fault. Simply by removing that pole my meshes would suddenly work again.
I absolutely love poles and loops. They make organic modelling fun, not arduous, they make my meshes neat and wonderful, not messy and convoluted. If you model anything, even if you just start with a box and sub-d your way down, you absolutely have to get in control of your loops. If poles and loops were two dudes I'd let them spit roast me and it wouldn't be gay. THAT'S how awesome they are.