As with Connor, I have agreed to write advice for Tim's five points. Here they are, reduced in size. Read the full versions on his blog.
1. Provide better exposition & storytelling to my players . . . ideally, I'd give the players an appropriate amount of exposition and colour whenever necessary.
You already plan to practice storytelling and you already recognize the difficulties of understanding a story that is told non-chronologically. You seem to understand the errors you've made in the past so I won't waste time covering those details.
Understand, however, that good storytelling comes with immense effort and difficulty. We're talking about an esoteric process, one that shouldn't be confused with the procedures necessary to create something even extraordinarily complex. Successful storytelling defies methodology, since that only serves to create the same story over and over. Storytelling defies inventiveness in that too often a story can become so ingenious that is fails to resonate with an audience that is, at heart, only interested in things that are applicable to themselves and their lives. Storytelling must be cunning yet candid; it must demonstrate skill and command and yet it must appear to be effortless. To manage that, lies must be sincere, truth must contain an element of suspicion and the effort to understand which is which must matter to the audience.
How is this done? No one has ever known.
There are others that seem to have known - and so we return to them again and again, taking the elements of their stories apart like automobile engines and electrical components, seeking to grasp their elements and how they fit together. We go back to writers of centuries past and writers who have explored every means of storytelling and we experiment ourselves and, yes, practice. But each storyteller, nevertheless, will have a unifying experience where it comes to success - we cannot explain how we ourselves succeeded. This is where we return to theories of muses, spirits that enter us and make us perform in ways beyond our human capacity to perform. This is where we find Galatea unable to explain the sudden truth of Pygmalion's human existence - and his equally helpless inability to control that existence once it begins to manifest its own purpose without dependence upon the creator.
Do not chastise yourself for the patterns of your storytelling or for the players' tendencies to run off with the stories you're telling in their own direction. There is only so much you can control. You can apply yourself, as best you can, to the message you wish to convey; you can concentrate, in some small part, about the characters you would like introduced to the players; you can trim the trivialities and steady your erraticism - but do not supposed that you will 'simply be firm' in your decisions. Having created the story, you will do well to ride it to its conclusion, recognizing that the story - like Pygmalion - will go off purposefully in directions you would not prefer, despite all your planning and efforts.
Where you can, manage the details. As Cesar says to Bart Simpson: "Now watch me. You grab the grape between your thumb and forefinger and gently twist it off and drop it in the bucket. Now you do it. [Bart plucks one off and drops it] Very good. Now do it a million times."2. Keep players more engaged during play. In particular, I'd like to hold my players' attention even when it's not their turn in combat or I'm talking to another player.
Here is something I did not write in my book. Early on in my running, when I was merely 16, I recognized immediately that there was a tremendous dearth of players willing to DM the game. I had four or five of my friends playing and we all knew perfectly well how gawdamn fucking hard it was to run this game. Right from the start, without thinking about it, without asking my friends for permission, I would shout at them to shut up without an instant's hesitation. If they wanted to be there, if they wanted to run, if they wanted me to tell them the stuff they needed to know and answer their rattled out questions over and over, regardless of whether I had answered those questions before, then they were going to sure as shit shut their yaps when I was talking, thinking or running the damn game. Or they were going to get both barrels in short order.
This did not, as you would think, result in people getting up from the table and quitting my game. The reason it did not was because it was very plain how much effort I was putting into it and how passionate I was. I talked fast and confidently and loud and I expected immediate responses when I required them. I expected my players to be passionate as well. This is something that continues to be plain in the games I play now. I'm working hard here. You're paying attention and your head is in the game or you can get the fuck out.
The result of this firm policy is that I do not have to spend 95% of my time controlling the players. In fact, I rarely spend any time doing so. My players do talk to each other while I'm running someone else, but it is respectful and considerate and doesn't disrupt the game. I don't have to force my players attention because it only takes a simple statement to get them back into line if things are drifting. You can catch me doing it in this video.
That level of attention that I get arises from a willingness to be intolerant of bullshit. If you won't shout or raise your voice or even close up your books and explain that you're not fucking here to run them like their bitch, they will walk all over you.3. Declutter my organization for the game.
The tools exist. PDF copies suck, they do not lend themselves to word searches, the same for OneNote notebooks. And yes, move some of that memorization onto your players. Explain to them that if they can't tell you their character's spells, their characters have forgotten how to use those spells. Don't take laziness as an excuse. You've got enough shit to remember at the table; if that mage wants to have spells, that mage can spend the time getting the spells organized and ready when you ask, "What's the range on that puppy?" This isn't rocket science.
I have a suggestion - but if you're not a touch typist, you'll hate it. I recommend that you physically sit down and retype all of those PDF documents manually into word. Not because that's the most efficient method, but because consciously thinking about the words as you script them out by hand will help you memorize both the content and the location where you put that content. Make notes on what things you want to expand while copying - even expand them yourself while you're at it. Put everything into word where you can search it by find/replace - and then teach yourself how to index material and write comments in word on that material. This stuff is way, way easier than you would guess - and steady copying will slowly put everything you use into a format that is convenient for you in times of stress.
4. Be less intense about D&D.
I can't relate to this. I will say that I can turn D&D off like a spigot; there are many, many people I have known for long periods of time who don't even know that I play the game. They know nothing about the blog or what I do - I simply tell them that I am writing. Others know what I do but since they have no interest, I don't talk about D&D.
Beyond that, however, for me, I intend to be as passionate about D&D as I wish to be. Where players are new to the game, I know this serves to enhance their experience and produce a level of wonder and mystery - as in, "There must be more to this game than I understand, look how much he loves it!"
But yes - if turning the dial down or off is necessary for other parts of your life, I understand. It's a matter of self-control to do that.
5. Get a chance to be a player.
Absolutely. Every DM needs at least some player perspective. If you can't get an opportunity among your group, then you're going to have to seek out an event or a club. The experience will be horrific, trying, largely unfulfilling and composed of time spent with the sort of losers who cannot get a game together with friends in a space they control, but it will be INSTRUCTIVE. In a capital way.
It will also give you perspective. You don't know how good or bad you are until you see how truly many of the campaigns in the real world achieve excrement-level.
If you have the patience to keep at it, you will find quality DMs running in public. Some people just get into that as a fetish. Unless your ass has been kissed by the goddess Nike, however, it is going to take time to wade through the dreck until you luck out. Pay attention to the players that drift in that direction and you'll be led to the promised land.
Hope that was helpful, also.