Ter: Since this is a FAQ as well, maybe a list of suggestions for how to make a good sign-up game?
Here would be mine (after having run several):
1) Creativity:
A good sign-up can't be like every other sign-up. Creating a super-team with everyone a superstar can only last for a short time, unless you do it really, really well. This can easily get boring for you, and for the people participating. Remember one of the original sign-ups did not have a 'team' at all, being just a batch of young Scottish strikers ("Create a Striker and Wear a Kilt") with updates as to their progress and success. Mix and match team, nation, and player types within your sign-ups. Not every sign-up has to be about winning everything.
2) Starting up:
When asking for people to sign-up, have a method and plan in mind. Most of the effort in creating a good sign-up come before you ever star the game. Do not expect to be able to enter every attribute for a player/participant! This will take too long, and doesn't help keep your interest much. Keep it simple. Names, Nationalities, Positions, and a few key attributes are plenty to get most people started and interested. When asking for sign-ups, specify exactly what you need (and what you don't).
3) Weaknesses:
Don't overlook this key aspect of players. If a player/participant has at least one (or several) weak attributes, the game is far more interesting. A striker with brilliant potential but with a short fuse and bad attitude is far more interesting (and realistic) than the perfect player.
4) Detail:
Game updates are great, but scores & ratings are only half the fun. News items, player interactions, contracts, disputes, this is what makes FM sign-ups more interesting. Create some suspense! Create some bitterness or feuds!
5) A picture is worth a thousand words. A video is worth a million.
Screenshots are a must. Regular shots of players and formations should be a natural part of any sign-up. There are loads of free storage sites on the web, use them. Videos, if you can pull it off, are an even better way to get your participants giddy. Goal Highlights, brilliant saves, etc, are all the coolest things a player/participant can see.
6) Update update update:
A sign-up must have regular updates or it will fall flat. This doesn't have to be everyday, but a regular pattern with lots of #4 or #5 will keep your participants happy. If you are going away for a while, let them know... or pass the game off to a temporary manager.
7) See how others do it:
Look at other sign-ups that are successful, and mimic their level of work. A good sign-up game is not easy to do, and most fail because they just take too much time. Don't start it unless you mean to carry through!
Alright, that may be cheesy, but you get the idea. wisla and loked probably have more they could add.