Posting this here instead of Coffee House because I'm looing for real tips - and I feel like it has a good connection to singing, vocal and voice-over techniques.
 
So, I'll be honest - I don't do much singing these days. My current line of work doesn't really have me running my mouth too much, so honestly I don't speak all too much in a day unless I'm with my family.
One thing I've noticed is that my speech articulation has taken a HUGE nose-dive - unavoidable vocal stutters, lack of power or clarity - lots of mumbling - and BIG-TIME struggles in my second-language and in some casual speech.
 
It sounds silly, but I really want/need to improve this. Especially to just be able to free my tongue up to speak freely!

Maybe some vocal warm-ups?
https://takelessons.com/blog/daily-vocal-exercises
 

I bought a vocal course a while back which included some excellent vocal exercises. After a few weeks my vocals were much better and I noticed a marked improvement on my diaphragm strength. It also managed to extend my normal vocal upper range by 2 tones and my falsetto range by around 5 tones.
 
Here's the basic routine:
 
For each of the exercises, start at the lowest comfortable note you can sing. Do NOT strain at any point as you go up the scales, and let your voice naturally go to falsetto (even if this is a lot lower than you'd normally switch to falsetto).
 
The scales should go up and down 5 notes at a time, e.g. F, G, A, Bb, C  then back down Bb, A, G, F. Then go on to G, A, B, C, D and back down C, B, A, G, etc. Each up/down scale should be done in one breath without a pause or break. Go up the scales until it's no longer comfortable - remember not to force higher notes during the exercises. With all the exercises, try to relax and let it resonate around your head, especially around your sinuses.
 
Exercise 1:  Humming an "N" sound.
Exercise 2:  Humming an "M" sound.
Exercise 3:  Rolling "R"s  (like your Scottish)
Exercise 4:  B's like blowing a raspberry - you may find it easier to press your thumb and fore-finger either side of your lips else you'll end up spitting everywhere.
 
Do these exercises 2-3 times, and notice the difference when you sing afterwards.
 
M.
 
 
 

If it's just general speech articulation you're looking to improve (and not singing specifically) then there's nothing wrong with just taking a novel and reading it out loud. I do this all the time and find it really helps my general speech articulation. Imagine you're a professional narrator doing an audiobook, so put expression into all of the sentences and do characterizations for the dialog. 
 
The main problem I have when reading out loud is paying attention to the rhythm of the words, and this is so important for articulating speech effectively. When you read from a novel while imagining that you have an audience, you have to really think about the rhythm and the flow, and it forces you to remember and draw upon memories of good speakers you've heard throughout your life. We've all had plenty of experience listening to actors and narrators. Pay close attention to them and try and emulate them when you read out loud. I have no doubt that this kind of exercise will help in your singing too. 
 
One thing I have discovered that really improves my speech articulation is nootropics. I've been taking a nootropic stack for about a month now and it's really improved my speech when I read out loud. I think it's because I have a mild ADD which causes my brain to get ahead of my mouth, and it affects the flow of words. But on the nootropics, I find that I can really focus on the flow of speech and take my time to articulate it correctly. Other's mileage my differ. 
 
 

#1 tip: back off the mic. 
 
Intelligibility comes down to consonants. Sometimes you have to over-enunciate them, especially in heavily-compressed genres. It's a skill you have to practice, getting your "K"s and "T"s just crisp enough without making anyone's ears hurt.
 
Once recorded, if the vocal isn't cutting through or the lyrics are hard to understand, an exciter + EQ can bring it out front. Intelligibility depends heavily on the midrange frequencies between 1 and 4 KHz.