I would recommend asking players not to abuse Declarations...and to house rule it if needed. The same applies to thaumaturgy (which is essentially a stack of declarations).
For starting out with Fate, I strongly advise against this. People have a hard enough time to wrap their heads around the aspect situation, without having to keep a limit in the back of their heads. You have to address the issue, if it gets out of hand, I agree, but to start out, I would welcome every use of aspects that there is, which usually is very little, because people are just trying to grasp the concept.
A good way to get people to do that is by returning the question to the player. In other systems, the players often ask things like "Is there a box I could hide behind?", or "Do I have a rope with me?" or things like that, you get the point. In cases like that, don't give them a yes or no answer, instead ask them back: "I don't know, is there?" It might be, that they don't get it, and then you can still ask a skill roll of them, or tell them to describe how the fact they asked you about could be implemented into the story in a cool way.
I had pretty much this happen in a game, which was one of the points where the nature of aspects really kicked in with my players. I had a couple of sorcerers performing a ritual together, and one of the players wanted to throw a spell into their plans from far away, because they couldn't get close enough to simply kick them out of concentration. I had described the scene in detail, and the sorcerers as being "focused on their ritual". The player asked, if he could use the fact, that they are so focused on the spell, that they can't really do anything else, which should give him a bonus on his roll. I told him to roll empathy, and being a "typical" action character, the social skills were at an absolute minimum. However, he decided to get an edge by doing a lore roll (he was part of their cult once), so he knew a critical point in the ritual where he could push them over the edge.
Another thing that is not that easy to see at first: One enormous enemy can be a lot easier to kill than a few less capable ones, due to action economics. One monster has only one action, and can only do one thing at a time. A group can stack aspects for the last character in line to tag, which can lead to ridiculously high numbers very very fast. Five Characters in a row, if all maneuvers are successful, can lead to one attack at +8 by the fifth character. There are no combinations of stunts and powers that will really lead to that kind of overkill, really.
The best way to plan a DF scenario isn't so much to have a plotline in mind so much as to flesh out your antagonists, their motivations, and their resources, and just work logically from there. That way, they can act and react to the PCs in a natural way.
Basically, play the antagonists like PCs--they have a goal, and you walk through their efforts to reach that goal.
Also, never, ever be afraid to modify your plot if your players come up with something good you didn't think of. They have a part in creating the story as much as you do, after all, and that includes occasionally modifying a villain's plan because the PC came to a logical conclusion. Players should always be rewarded for creativity, initiative, and figuring stuff out on their own.
I will completely sign this. There is no "The players are killing the plot" in Fate. Whatever the players do IS the plot. If there is an easy way to kill the villain, fine. There is always a bigger fish. Or the easy way is cutting some corners that will bring in other factors, that were totally unplanned before. One of my players in my pbp game took an NPC that I had merely thought to be a little more than background noise and included her into his backstory on the fly, making it one of my favorite characters and giving her a regular appearance I had not anticipated.
