dip it in a batch of quick setting cement. that'll make it pretty hard.
I was actually going to suggest some kind of polymer. Concrete may turn out to be a little rough, and since it's pretty porous, it is prone to breaking.
Seriously, I have to agree with Ax3C and Dr. Death here- and this seems to be coming up more and more lately. It's not a medical forum- it's porn! And sure, we discuss a few more things than that, but we can't give medical advice.
Now, if you wanted to blame your weak erections on US President Bush...
H