All fursuits start out as blank slates. It is up to the suitmaker to decide the personality of his creation. Consider the audience and the performer when deciding on the character's personality.
For example, an angry character with sharp fangs would not be appropriate as a children's mascot, but would probably fit right in at a fur con.
Try to give your creation at least some sort of expression, otherwise it can turn out looking very dead and spacey.
A smiling character is fairly versatile and good for general use (ie: kid's parties, cons, general costumed mischief). When making your mask's expression, pay attention to how real people's faces work. When you smile, your cheeks are raised, as are your lower eyelids, generally.
Adding on eyebrows is also a good idea, but make sure you follow the general curve of the top of the upper eyelid... if it's a really drastically different angle, it can be really confusing (ie, express two different emotions at once).
And of course, add a little sweet smile. :) I personally prefer stylized teeth instead of potentially-frightening realistic jawsets.
Just be careful not to make it TOO happy... hehe. :)
A little shy smile can be very endearing. The brows lift a little in an almost apologetic look, and the smile is tight-lipped. Less cheek action than the full-out smile.
Crafty smiles are perhaps made easier with 3D following eyes (there are a few different tutorials on this in the memories!) because no matter where the viewer is, the mask will be peering over at them out of the corner of their eye with a conspiratorial grin on their face.
Lower the brows and add on a close-mouthed grin. Ears can also be folded back to further the sly look.
Glamorous fem eyes can be heavy-lidded and lined in black- as if the character were wearing mascara and eyeshadow. The eyebrows should be finely shaped (consult fashion mags if you're having trouble), and it really helps to go nuts with eye detailing- that just makes them that much more captivating. :)
Angry/Stern eyes have a very furrowed brow. The top of the eye recieves more tension than the bottom, unless the character is REALLY angry, in which case the whole face contorts.
this image got lost ;-;
The confident/challenging look is really common in professional mascots. The above image has a few ways of achieving that effect.
Insane eyes are rarely done, but are fun looking when done well. You can purposefully fudge the symmetry a bit with these, especially when it comes to pupil/iris size. Having the eyebrows on different levels helps add to the look of delirium.
Eyelids can be pushed up far and the mouth can have a HUGE grin, especially if it has a lot of snaggly teefs.
Demon/Monster eyes seem to be done best with taxidermy eyes, making the character look totally inhuman. Really small pupils will make the creature all the more intense.
Both lids and eyebrows can be lowered for a menacing stare.
Relaxed 'cool guy' eyes can have visibly lowered eyelids and relaxed brows. Careful not to make him TOO relaxed though...
"Wanna buy some deathsticks?"