from Chuang Tzu (rephrased),
"When a fisherman is out on the lake in his own vessel, and he bumps into an empty boat, he will consider it an accident and think nothing of it. However if there is another man in that boat he will become irrate and shout abusively, believing it to be the fault of the other man. A pure man becomes like an empty boat."
Goes something like that anyway. Basically, alot of fights between young males is about ego and intimidation (basically trying to impress potential mates or increase their position in a hierarchy). It is difficult to second guess someones intensions (some people are just annoyed for something you've done, where an apology is useful, others want to fight and will see an apology as a sign of fear). Break out of the whole aggressive loop. If there is a problem, just be honest, rational but neither defensive nor aggressive. Be prepared to do ANYTHING and thus have no fear of a fight, nor any desire for one. This will help you to choose the appropriate action.
On a more practical level, ignore eye contact (unless you want a fight), but don't shy away - just get on with what you are doing. Interestingly with many animals, often strong males will make eye contact which may be a precursor to a fight. However, a child can make prolonged eye contact with a adult male and the male will just ignore it (because it isn't even considered a threat).
Thus, if you wish to break eye contact without looking weak a) don't feel weak! b) realise its just superficial bull-shit c) just get on with what your doing d)
Yawning can be a reaction, but often that is seen as threatening behaviour (yawning may derive from a display of teeth (main weapons) in some primate situations).
As an aside, I'm a big believer that acceptance of death and defeat is essential to a fuller understanding of martial arts. Basically, you have to do what is appropriate when the time requires it. Pretty much anyone can kill anyone else (Takeda wouldn't let people he didn't trust within 3 yard of him).
As far as pre-emptive strikes go. I am all for them, but you need to make a judgement on whether you feel that a physical confrontation will actually occur. As you say, if you know they are definately looking for trouble (and it has to be with you), go for it. However, a word of caution: I once pre-emptively struck someone in the eye with my finger and it it looked to really have damaged them. It completely enraged them, and I actually felt weakened by the guilt which resulted me getting a bit of a pounding (not sure if I permanently damaged their eye; there was blood coming out of it).
A better technique is to have a buddy looking out for you - if the aggressor approaches, your buddy sneaks around the back and chokes them out (then its over in a matter of seconds and the aggressor won't even know what happened!) - I've tried this very succesfully twice.
P.S. I'd agree with previous post saying trust your instincts as well. What many people don't realise is that a real fight starts with a difficult and rapid assessment of the situation with limited information; that's why I would never be arrogant enough to judge someone else's response to a potential attack. Most of all, don't kick yourself afterwards (I don't, and I may have blinded someone and got a kicking, all at the same time
P.P.S. if this is regular intimidation by the same person, I'd actually go for an all out attack - just the effort you put into it may help to curb the situation in future.