I recently acquired the Space Shuttle set, #10231, and find that it isn't a staged build--one with sequential bags. This leaves me at a bit of a loss over how to organize and search through 1200+ pieces while putting it together. What technique will allow me to more easily find the needed part during construction?
I recommend getting 15-20 small-medium tupperware containers (you can get them pretty cheap now). Separate the pieces in a way that makes sense to you. Tupperware has flexible sizing and they are transparent, making it easier to identify pieces from the side of the container.
You don't have to completely put each individual piece in its own bin. But, for example, you could put 1x1's of all colors in one tiny bin, all transparent pieces in another, etc.
The important points on choosing your bin contents is:
The first rule and the second are often conflicting. One thing I recommend is to keep even-length and odd-length pieces separate. Don't put 3 and 4 longs (of the same color and type) in the same bucket, but 2 and 4 is fine.
Half an hour sorting the pieces at the start can save much more than half an hour in the build, as well as making the build more pleasant and fun.
You'll just need to separate all the types of pieces so that you can easily access any piece you need. I believe this will work just the same for the 'jumbled up' case as well ;)
What I did for the Death Star minifigure set (that has no numbered bags in it at all!) was to make long chains of identical bricks regardless of colour, (a system based on shape not size). Although I do put similar shaped pieces of different sizes together especially with LEGO Technic pieces, just overlap them slightly to make it easier to break up for use and one can stack them in a box for storage.
Again it's easier if one sorts out the bricks before a long and massive build as it saves time hunting down that elusive part and getting "brick blindness" in low light levels.
Building by day light is essential if the plans are bricks are hard to see. To avoid cramp, try to build on a table and not on the floor or on a bed as this can slow down the build.
Take short breaks between completed sections or take it day-by-day or one can be up for ages. Also as an experienced builder, I find one can get quite cold sitting in one position for a long period of time, so hot drinks and the central heating on full can help with concentration that and the phone off the hook helps too...