Also known as the lawn mowwower by my three year old son.

Before I explain what you're going to do, I want to tell you why this works: 

To sing you need to have a nice steady stream of air. The air makes the chords vibrate (SOURCE), the vibrations bounce around your mouth, throat and head (RESONANCE) or, to simplify further, you MAKE the sound then SHAPE the sound. 

So let's start with MAKING the sound.

1) Creating a steady stream of airflow

I have a few favourite exercises for this (the straw, the lip trill and the blower) but this is today's top choice. 

These exercise are my own - some are adapted from an ENT website because I find they work really well.

1) Yawn. Feel the throat open and the soft palate lift. Let that yawn out in a soft deep throated sigh. Hmm.Feel the lovely vibrations in the base of your throat, down your chest and into your sternum.

2) Now, around that lovely low note, make a strong sustained sssh sound with the vowel OR vocalised in your mouth (Say or then close your mouth in a ssh and continue the sound)

Feel the vibrations. Let them fill the resonating cavities around your eyes and cheeks, let them flood through your mouth and throat, down your chest and into your sternum. Make your self a vessel for sound and enjoy. Do this for as long as your like. It will be engaging your breathing muscles without you having to think about them (feel your stomach tighten as your make the noise). It will remind the muscles of the job they're about to do. 
Look at your neck, it will be widening and relaxing as the muscles release despite the flow of sound.
Focus on your pharynx or mouth. The closure at the front will allow the sound waves to bounce back and support the chords so let are not straining to control the gush of air from the lungs.
Let your cheeks relax, maybe puffing out slightly 
Let your jaw relax as if there is a nice open gap between your wisdom teeth.
Let your tongue hang. That beast can get tight and now is the time to let it loose.
Keep going until you literally feel warm: warm and buzzy, loose and ready.
Go as low as you comfortably can to let the larynx drop and relax. Watch it lower in a mirror.

After this you can take the same sound up and down, sliding smoothly making sure to keep the air flowing (you should always be able to hear a steady ssssh with this sound). Try to feel the vibrations everywhere as you move, keeping the chest resonance as you go high and the cheek bone resonance as your go low.

When I do this exercise, I feel a little like how I imagine a monk might feel. I could feel a little silly, making noises like my three year old son behind a "yawn mowmower" but just give in and enjoy the feeling of being a vessel for sound. Letting your instrument play through you. I promise it works.

One of the things I found most frustrating about being a singer was the inconsistencies day to day. Why, some days, was it so easy, yet other days my my voice felt like an unwieldy beast, pushing through a swamp to make a sound? 

I used to worry that those days might be a sign of vocal harm and then came the downward spiral: afraid to push the voice, I'd hold back, tighten, stop breathing, panic! 

When you have an off day, it's important to understand whether it's a sign of vocal misuse or just.... an off day!

Like everything else in life, some days the voice tries to hibernate. We've all had days when our brain feels slower, when our limbs feel like rock and doing exercise feels like some kind of torture. Sometimes it's plain tiredness, sometimes it's to do with emotions or, for women, that time of the month (yes, it affects the vocal chords t00. The water retention caused the chords to thicken making high notes harder. Here's an article why http://www.totalsingersupport.com/singing-your-best-at-that-time-of-the-month/)

Assuming that you have ruled out any vocal problems (ie. if your voice was fine yesterday, but today it seems muzzy/stuck/tight) then what can you do? 

We embrace that, in life, we all ebb and flow, we have good days and bad. If this is your career then you need to manage that. You all know to get your sleep and eat healthily and drink your water. What I want to talk about is how, on those days when it's just not really happening, where you're under the weather or just "under", what can you do?
Have you ever felt rubbish - flat, tired, foggy - and then something has happened that woke you up, that shook the fog away? Let's look at some of those things so you can use them on a day when you need it!

1) Running! Yes, might be the last thing you want to do but exercise WILL wake you up. As a singer, I always hoped that they'd be a dance call first because (even though I have two left feet) it would wake me up and make me sing better. It works. Your heart starts pumping blood to your muscles, happy hormones flood and you're ready to go.
2) Music. Loud. On the way to rehearsals, I always listen to music. During Aspects of Love, I had two albums that would get me ready: Too Many Dj's and Hairspray the Musical. If it gets you buzzing then don't be ashamed. Find whatever works for you.
3) Coffee. Uh oh. Am I recommending caffein for singers? I love coffee. It does dehydrate so match with lots and lots of water but sometimes a big coffee is what is needed.
4) Walk. If you're too tired for exercise, get off the tube early and walk to where ever you need to be to sing. Put your headphones in and walk with music.
5) Meditate. My favourite. Something there is too much weight from the day to day stresses. Sit. Observe your breathing. Feel what you're feeling. Count down from 100 and go back to the beginning if you lose count. Let your breath relax and your mind calm. (walking with music is mediation too - anything that still a chattering brain)
6) Chat to someone who makes you excited. You know, that friend who gets your buzzing. Call someone and get out of your own head. 

After that, be kind to yourself. Life isn't constant, it ebbs and flows, joy passes and sadness too. If you're down then you'll be up again so give yourself a break if today isn't your best day and do the best you can do for today.