I love to hear everyone's planning techniques! I must be crazy... but this is literally the only place I can talk planning and not have to watch my audience's eyes glaze over.
First, prioritize! The most expensive items are probably the best place to start. After that, pick one topic and focus on it. If you're trying to decide on a rental car or Disney transportation, don't get sidetracked with hotel info or DDP discussions. You don't have to make decisions too early in the process (except for hard-to-get ADR's), but sometimes it pays to "overplan". The rental car is a great example - if you find a good deal, book it! If you decide to cancel later, there's nothing lost (not true of airfare or hotel, though).
Whenever I'm browsing DISboards or looking through a Disney-related website, I open 2 files on my computer and leave them open. One is my "Tip Book" in Word. The other is a crazy Excel file. I can't function without them. If I tried to keep all of this in my head, I would explode!
My Tip Book started out as a bulleted list of tips. When it grew to many pages, I organized it into categories. When that got cumbersome, I subcategorized it, entered some "headings", and started using the "document map" feature in Word, which is basically a clickable table of contents. It's now formatted into small 3.5x5" pages that fit into an index card binder. It's 141 mini-pages long. I don't bring my guidebooks to Disney, but I do bring my Tip Book! It's been an excellent resource, as it was written specifically for my family, the parks we visit, the hotel we're staying at. And since I took the time to "do it right" for our last trip, this time all I'm doing is updating the details. I'll print it out and punch holes about 1 week before we leave.
Toward the end of my planning last time, I buckled down and worked out daily itineraries. I included those at the beginning of my book. I really couldn't have figured out the itineraries until I was a few months into planning. Even using Touring Plans, there are extra details that get tricky to work in.
My Excel file is a hodgepodge of critical number crunching and fluffy to-do lists. But it's all in one place! On one tab, I have my overall "budget" assessment - listing airfare, hotel charge, rental car, tickets, DDP, other food, etc. I then check different weeks and different hotels, cross-reference the flight costs, mess around with adjusting a day here, 2 days there... it's convoluted and insane. I had 15 different plans mapped out for our March trip. I don't see any way to make it easier! But spending that time has saved us $1000 already.
Like BlackBeltDisneyGirl, I use another tab to list all the critical data for each day, each park - who has EMH, what the crowd levels will be, which parades and fireworks are scheduled. I color code the good and bad (red=bad, green =good). This helps me figure out which park we'll be in each day, since we're not hoppers. This also helps with ADR planning.
Another tab is my DDP assessment. I spend far too much time evaluating the DDP, but at least I KNOW in the end that I've made the best choice. One thing I've learned is that this assessment has to be redone for every trip. Hooray!
I have a screwy packing page too. I write down which specific bags are coming - all bags, carryons, checked bags, even my purse. Each bag gets a column. Then I list what will go into the bags. Every last item! When I read a tip to bring a power strip with all the electronics cords plugged in and labelled, I immediately added "power strip - loaded" to one of my bag columns.
I actually have a separate tab just for toilletries. There are so many rules, and they seem to change so often! Rather than write down the rule, I just categorize each item - dry carryon, 3-1-1, or checked bag. E.g. - I put a sample-size sunscreen in my 3-1-1 and a full-size sunscreen in my checked bag. I DON'T just write "sunscreen" on my packing list.
I have another tab "To Buy" and yet another "To Do". Whenever I buy or do ANYTHING, I check it off - I don't just delete it, since I may want to have that record for the next trip. All items purchased go straight in my Disney box. Then they don't get lost or "repurposed" by dh!
There truly is SO MUCH to consider, remember, and decide. There are also so many options at any given moment. Keep yourself as organized as possible, and it will be less overwhelming. Take a break once in a while - read a novel (a short one!), go to the movies, have at least one non-Disney conversation every day. Also give yourself permission to "make mistakes". The magic of Disney cannot be planned. But a well-planned trip can offer many opportunties for the magic to happen.