Eliminate noise in your space
I am a noise freak. I kind of can't move on if I notice that there's extra noise, either in my production space or in the finished product. The best way to eliminate noise is always at the earliest point possible. Eliminating noise after the fact by messing with a digital file almost always comes with a cost: a diminished sound of whatever is on that track. If I take an acoustic guitar track that I've recorded and filter out hiss (the result of poor environment or mic placement), i am likely also altering the actual sound of the guitar in some way. Not always the best case scenario.
So, in my recording space, it means doing everything I can to get as pristine a signal from my mic as possible before I ever record anything that I wish to keep.
Machines of all types make noise. The most likely noise maker in a project studio like mine is my DAW. That little machine usually has a fan inside and that fan moves air (kind of the point) and that simple fact of physics means noise. My laptop, that charges on the desk next to my mic boom, makes noise too. If it's on while I'm recording with the mic, there is a very slight but discernible hum somewhere in the 60-120Hz range. Audible, but low level. Once I made that discovery, I started moving my laptop to the floor anytime I'm recording with the mic. The last thing I want to have to do is always run a high-pass on anything recorded using the mic. Unless of course it's something that I'd choose to do anyways. But that's kind of the point. I want the choice.
When I'm gearing up to record, my studio undergoes a little bit of a transformation, of which I'm including a few pictures. The cubby where my Mac Pro is housed gets covered by a thick couch cushion and thick blanket. I try not to keep this there for very long, just as long as the session, because I realize that the fan inside the computer actually has to work more with less free air hitting the computer. But doing this reduces the machine noise emanating from that corner of the room significantly.
I also hang a large duvet from the door to the room to absorb extra noise and eliminate reflection. With the duvet off, I'm faced with a large reflective surface right behind where my microphone normally faces. So I toss the down comforter over the door and just that extra 30 seconds of preparation significantly improves the recorded sound.