>> I'm a very organized person in every other aspect of my life ... except gardening.
>> although I have good intentions of being an organized gardener ...
I agree with Lin. Maybe "good intentions", but in practice I'm lucky if I get the seeds in and mulch laid down at the right times. Work, weather, obligations and laziness (and sometimes illness) usually interfere with plans.
As others have said more gramatically, ya gotta plant what ya got, wherever ya got room. I'm gradually creating raised beds, but there are always more seeds than space. Until recently, starting seeds inside or direct sowing was an iffy process, and whatever survived would be what was planted.
Really, my "plan" for the first few years was to see if I could make good enoguh soil for ANYthing to grow. Then, it was "can I grow this?" ... "can I grow that?" Enthusiasm and stubburness became the drivers: Delphiniums took 2-3 years and Penstemon are taking around 4 years to get something to survive.
I understand that, in theory, I could look ahead months or years and then start things to satisfy the plan, but in practice, each time I have an open weekend and it isn't raining, I look around for what can be accomplished that weekend.
Then I planted some perennials (one year bulbs and lavaterra, then Salvia, snapdragons and poppies). That limits your plan-ahead options even more! I did try to plan places where they would look good, and even a shred of thinking about keeping harmonious colors together, but only one out of four things thrived. Maybe the ONE Mikado poppy and the ONE Grape Opium poppie that sprouted and survived will re-seed. If they do, I'll find out how they look there ... and modify the plan.
Talk about planning! I "planned" a spot for lavaterra. Since they were big, deep-rooted perennials, I dug that bed DEEP, drained the ever-living heck out of it, put all my compost there, fertilized and watered. Well, it turns out that the worst possible thing to do for lavaterra: they sprawl and flop and, in my case, became covered with ants. despite the "plan" and lavaterra's deep roots' dislike for moving, I had to uproot them and move them worse soil in a bed where I could withold fertilizer and water. They're doing better now, than you!
Then I planted a clumping bamboo where I "planned" to have a tall, small bed of bamboo. So it droops gracefully, isn't tall at all, and takes up about 6 times as much area as "planned".
>> planning only works if you have a handle on all the elements
I agree! If you can't forsee the future, you can't plan ahead. Maybe I "retro-plan": try for one thing, get something else. Try again. If at first you don't succed, that's gardening.
One thing I AM trying hard to plan: I want to save clean seed from many Salvia species, and poppies.
So I planned to plant only one of each species in the front yard, and one of each species in the back yard. Maybe half of the varieties I tried to germinate sprouted well in trays.
And, my germinating trays produced more S.coccinea varieties than anything else. 'Forest Fire', 'Lady in Red' and 'Coral Nymph' survived germination and cried out for bed space. "OK, I thought, what are the odds that both of the second two will survive the slugs, weather, and my un-green thumb? I'll plant them as far apart as I can in the back yard (S. coccinea 'Forest Fire' was already doing fairly well in the front yard).
And then, of course, 'Lady in Red' and 'Coral Nymph'grew better and produced more seed than any other variety or species of Salvia. And I have to mark those packets "may be somewhat cross-pollinated with ..."
Now, watch. They'll probably be the only Salvia that reseed or over-winter and I'll have to move one of them to get clean seed.
For me, "planning" is replaced by "live and learn". Maybe some year I'll know ahead of time what will or won't succeed, and have enough bed space that I can clear one whole bed out and "plan" what to grow there. Maybe!
I look at landscape magazines and ooh and aah at designed gardens and think I'm going to do that! I buy plants intending them for a particular place and then just end up sticking them here, there and everywhere. I've had plots originally designated for Roses that eventually end up with Roses, Echinacea and whatever else gets thrown in the mix. I've had plots for perennials that end up with vines, perennials and annuals. So, although I have good intentions of being an organized gardener, I always end up being the go with the flow kinda gal. 
with Allison. However, I admit to doing some planning as far as where I'm going to put some of the plants I select for our honey bees depending on when they bloom. I also have one bed where I try out new plants to see how I like them. If we get along they usually get put somewhere else for next season; if we don't well they just don't get a second chance.
My minds eye has no vision. I buy what I like and put it where I can find a spot. If it doesn't look right, I dig it up and move it. What I need is a new 'minds eye', one that Works, cause the way I'm doing it now is too much work! 
