When I argue for abortion, I tend to sidestep the "embryo is a human too" argument, just like I sidestep the "animals have rights too" argument of vegetarians. That's stepping into ideology, and I think it's much more effective to argue the practicalities of the matter than to convince someone to change their belief system.
First, being a parent, and even being pregnant, is profoundly life-altering - and it's almost always the mother who is saddled with the changes.
Second, abortion may be gruesome (especially when pro-life groups put up ten-foot-high pictures of aborted fetuses all over Sproul Plaza), but so are fetal alcohol syndrome and other mental and physical disabilities that are induced by drug use, recessive traits, or generally not caring enough to be healthy during pregnancy. When I saw those posters on Sproul, I wanted to counter with pictures of major birth defects that are due to the mother not caring about her unwanted pregnancy ... maybe pictures of child abuse also. There are too many unwanted children in the world.
Finally and most importantly, women have been getting or inducing abortions for centuries. They recognize how life-altering having a baby is and will avoid having one at any cost. If abortions aren't legal, women who do not want to carry a baby to term will seek illegal and often unsafe alternatives, just as they have throughout history. (It is not reasonable to simply tell people to abstain from sex - it's too much against human nature. Utah's sex ed. program sums up to "abstinence is good" - the teachers can't even give more information unless specifically asked by students - and when I was in high school, Utah had one of the highest out-of-wedlock teen pregnancy rates in the nation!)
Arguing whether abortions are "right" or "wrong" will not change the fact that women will get them - but making abortions illegal will just increase the number of women who die horrible deaths from unsafe abortions. Coat hangers or bleach in the uterus, anyone?
*steps off soapbox*
I like Micheal Moore's comment about abortion: men have no place to say anything against abortion, since they'll never have to actually deal with being pregnant!