When dealing with mostly rational people, a general-purpose counter-argument for the "yuck factor" can be "Your personal emotional discomfort with some developments is not a valid argument against scientific progress." This makes logical sense and will appeal to those who prefer logic. However, those from the "Yuck factor" may be focussed on their emotions over logic, and no amount of logic will counter their emotions.
To counter emotions with emotions you can do something like, "Sure transhumanist medical interventions are scary, but not as scary as deteriorating throughout old age until we die." (a good argument for cryonics - "You think it's repulsive to lie frozen in liquid nitrogen. Do you find the images of worms eating your decomposing body and bugs crawling in your half-eaten brain more attractive? You have issues then.") Or, "Sure people who want to transform themselves to look alien seem strange, but at least they choose their own strangeness. It would be stranger for society to force people to live their entire lives trapped in a body that dislike just for other people's viewing pleasure."
Likewise, religious arguments cannot be countered by scientific facts. If someone says transhumanism is against god, arguing that there is no god is not going to help. Instead, one may find it more useful to say "If god made me this way, and gave me this brain, then he probably expects me to use it." Or, "if god want to take my soul when I am dead, I doubt a little cryonics freezer is going to prevent him from doing so." Or, "if reincarnation is true and cryonics doesn't work, then I am sure my cryonics freezer won't last forever, and sooner or later my body will return to the earth and I will be reborn again."
There is one great thing about negative responses to transhumanism - they aren't original. In fact, there is at most about 20 standard knee-jerk reactions that are automatically evoked in people. Among them are:
"Doesn't He want us to live up to our fullest potential? We can have time to truly redeem ourselves, and build holy lives, before we are judged, because none of us are perfect.
I believe He loves us as our Father. Why wouldn't He want us to have th every best?"
"I think trying to help people is one of the ways we are in Gods image. I just can't stand back and by my inaction let millions suffer and die, I don't think God gave us the ability to fix this if we wern't meant use it. "
"C.s Lewis beleived that death is not natural, because the bible says that before the fall immortality was our natural state. And death is a curse on the living. God rewarded many of his followers faith with long life, like job who chose to stay in this world"
"Jesus was a great healer of suffering, he wouldn't want the wisest of us to suffer"
"I guess it is against nature, like glasses and fire are against nature. But if you look at it another way making things better is just another part of being human, arn't you glad that we don't practise cannabilism or that our courtships don't involve clubs. That doesn't mean we need to hurt the earth when we improve things though! Truly civilised technologys can let us live with nature, and even repair nature. We need to reach this stage before global warming and habitat destruction kill of most of our planets ecosystem. Luckily where nearly their."
"The fact that the rich are greedy can work in our favour. It will be like penicillin, in the beginning the rich wanted it so much they footed enourmous bills, this helped it develop. Eventually it was cheap enought that it could save the lives of those who really need it. If we made sure these were cheap before they were available it would cost us a fortune in tax. Let the rich foot the bill I say."
The good thing about them is that since we already know what our potential opponents will say, we can prepare a good response. And since our opponents haven't really changed their reasoning over the past decade or so, we can hone our responses to perfection. May be we literally should learn perfect witty and "original" responses by heart and be able to recite them when needed.
We need to identify all those difficult challenges from the other side and prepare strategies for the other side to use against us, and then be prepared to respond to those. The other side may bring up higher shock values to make us look bad and we must have a response (that would give us confidence).
"Instead of defining life by death we can define it by goals and milestones, or retirement, or replace death with a 10 year holiday at 80" "That way we don't have to cut life short just to define it"
at Fight Aging! - people have no enthusiasm for healthy life extension precisely because they think it that a longer life necessarily means more infirmity and disease.
"Instead of having endless hip-replacements, cancer treatment waiting lists like we do now, we want a fountain of youth so you can opt out of being senile"
The assumption of boredom, I think, is connected to another common misperception: that anti-aging technologies imply people will be forced to use them to live longer than they would have wanted.
The emphasis may be that real anti-aging medicine would provide you with an option that you didn't have before. No-one is forcing you to take it, and it's your decision.
Or this could be countered with a powerful image of all the things they could do with their life givin enougth time.
"If you become bored you could become a doctor, read all the book ever writen, explore fiji, the moon, or meet new people. Come to think of it you could try getting old, or take anti-boredom pills too"
"I think that killing of our elders isn't the best way to control the population. After all - the more people living truly fullfilled lives the better. Expansion into space, or sustainable technologys could let us sustain huge edens of people. I think what we should do is wait untill we are mature at 100 or so, before having children, then the whole community can lend a hand to help raise them with love"
A good example of a mixture of techniques with some fairly insightful responses is http://www.gen.cam.ac.uk/sens/concerns.htm