This answer varies greatly according to:
1) How many players you have
a) fewer players makes it easier to compel one repeatedly and stay with the story line
b) a bigger group seems to benefit from situational compels; asking "who has an aspect that they think would compel them to [save the princess/slay the dragon/do X] here?" is a good strategy.
c) Bigger groups make personal agendas like "looking for my lost sister" really hard to compel while keeping the story universal.
d) 5 players means you're keeping up with 35 PC aspects. 8 players means you have 56! You're just NOT getting to all of those.
2) How well the characters blend together / were designed to mesh at character creation
a) IF your nerdy scientist type has all his aspects about investigating things, and the werewolf is all about hitting things, then some scenes are only going to hit one person. If the nerd had an aspect of "help the werewolf", now he gets compelled in all the fight scenes.
b) IF you have players with completely disparate main agendas, it's harder to compel them at the same time. IF those last two phases reflect a bond between the characters, then it's easy to compel one character onto the other's agenda. IF one has aspects about being a do-gooder and the other about being selfish, it can be difficult (but fun) to compel both simultaneously.
3) If you write scenes around the characters' aspects
a) several blogs (rick neal, ect) suggest planning scenes by linking them to character aspects. The simplest (I really pared this down from the blog) version is to put all the PC aspects onto scraps of paper, then draw a few from a hat as you script each scene and specifically design elements in the scene for that aspect compel. Then, WRITE THAT DOWN so you remember to compel.
b) by choosing aspects, the players have told you the kind of game they want to play. IF you write a scene that includes one of each character's aspects (easy with a group of 4 or less, again, big groups complicate this), then you'll usually find that you end up compelling much more than just the one aspect, because you're hitting on what the players wrote their character around.
4) How readily your characters self compel
a) this is pretty obvious, but again, way more important in a bigger group. Training new players to this is key; after a while my players would choose to fail in a scene to earn fate points. "Hey, it would be really inconvenient for me if I couldn't get this info out of his computer. Can I have a compel on my high aspect [wizard]".
5) How easy the aspects are to compel.
a) also pretty obvious, but if you write your aspects in latin or haikus or whatever, and it's not really obvious what that aspect is for (yes, even if we had a conversation about what you wanted it to mean), then it's damn hard for me to compel. Really good blog advice on this one, too (can't remember if this was Rob or Rick), about having the aspects be kind of like catch-phrases for the character. IF the character would say it, or if it would apply to the character, then it's a compel. (I think this was Rob. He's at
http://rdonoghue.blogspot.com).
b) equally, the book and every blog give a ton of advice on making compels both positive and negative, so you can both compel it and use it. Here's my own personal take on that:
For every point of available refresh you have, you can have one completely positive aspect, OR one obscure/ mostly irrelevant aspect (looking for my lost sister, in a large group). Meaning, a wizard with -1 adjusted refresh really needs 6 out of his 7 aspects to be things that I can compel. Meanwhile, the pure mortal with an adjusted refresh of 4 can really afford to take 4 completely positive aspects. He's going to be starting battles with plenty of FP, and refreshing each game easily. If his other 3 aspects are really broad and focused around his character concept, he'll earn those easy enough. He really needs to focus on outlets, though, making sure he can spend those points readily.
Never, ever, ever take a completely NEGATIVE aspect. It's your job as a player to figure out how to use your weaknesses to your advantage. "Blind" is a horrible aspect. "Keen-eared, blind old seer archetype" is immensely useful.
c) if the players wrote their aspects to cover all three categories of combat (physical/mental/social), the aspects can be useful in most situations. IF they wrote all physical, then it's hard to compel them in a social scene. If they wrote one aspect for each type, they can run out of ways to compel in each scene. So I try to encourage players, for EACH aspect, to list how it would be used both positively and negatively in at least two out of the three types of combat. For instance, I had a player with the ultra-mature aspect of "big boobs!", but actually this ends up being pretty easy to compel or invoke in all three types.
Wow, this was long. Hope it helped!