It will be difficult to see the solution when frustrated with the problem. I agree that walking away from the puzzle could be very helpful. The person might also try changing their perspective. I often find when working on a challenging puzzle that it helps to change positions. If the person was seated at the bottom of the puzzle, they might try standing or moving the top of the puzzle. Another strategy might be to group the unmatched pieces into patterns or colors. For instance, group all the clouds or grass pieces into piles. That way when you are working on the clouds you only have to look at the cloud group rather than all the unmatched pieces in the whole puzzle.
I agree with #7. The best thing to do if the person is frustrated is to leave it for a while and go away and do something completely different. Then, when they have calmed down and are able and willing to approach the jigsaw again, they can return and try and break down the large task into smaller tasks, maybe starting by locating all the edge pieces and working with those.
If frustrated, they need to walk away for a little while. Sometimes one's frustration will work against them. After a cool off, they need to find a way to break all of the pieces into smaller groups--color, if "female" or "male" pieces, even by section.
In addition to looking for the straight edge and assembling the frame of the puzzle first, look for complimentary shapes that might fit together (these shapes tend to repeat in most puzzles) and color schemes that are similar. By sorting through the pieces first and assembling smaller parts to start with, the larger task of completing the puzzle becomes less daunting.
I agree that looking for smaller patterns would be helpful. My wife is currently working on a puzzle that consists of a map of the area in which we live. She has already found most of the straight-edged pieces and thus has much of the edge of the puzzle completed. Since the map features rivers (in blue) she can now begin to look for pieces with parts of blue rivers on them. Some of the pieces feature symbols for railroad tracks; she can now begin to group those pieces together. This puzzle, too, also has the names of various towns on it, so now she can begin to look for pieces with lettering on them. I would not have the patience to do this, but my wife enjoys this sort of thing!
I love jigsaw puzzles and I've been able to get my older daughter (now 9) to love them as well. What I did to make the problem easier for her (in addition to what Post 2 said) is to concentrate on details. I would have her look at a hole in the puzzle and determine what shape is needed (how many knobs, for example). Then I have her look through the pieces that might go there (if we had already sorted them by color or other feature) and look for pieces that could possibly fit.
As Post 2 says, it is all about breaking a task down into smaller tasks so it does not seem so overwhelming.
I agree with post #2. Jigsaw puzzles require people to see both the forest and the trees, and it's easy to let one obscure the other. I think the best suggestion is to begin breaking the puzzle into smaller chunks, and organizing it as the previous poster said, by color, is perhaps the best way. There is nothing wrong with looking back at the overall design of the puzzle to make things easier either. While strategies like these are important to putting together large, complex puzzles, I think patience and perseverence are equally crucial.
Encourage the person to search for common patterns that might help in sorting out the puzzle pieces. If the puzzle has straight edges, put all the pieces with straight sides in one area. If there are large areas of a common color or pattern, group the pieces showing that general characteristic together and start working to find pieces within that group that fit together. As with many seemingly intimidating activities, the key is to break it down into smaller parts and tackle the smaller tasks one at a time instead of allowing yourself to become overwhelmed by the whole.