Don't you love how forum topics usually always get off topic somewhere along the line? Guess the whole world is now ADD....
Anyway, we have the same problem with our 11 year old and have for the longest time. However, he has at least told us the reason. He simply thinks it "takes too long" and he considers it "work" and only thinks he should have to play and do whatever he wants to.
Plain and simple, he is somewhat lazy and really does not care at all about hygeine - appearance, smell, health, nothing - it is not important to him. He has been diagnosed with ADD and ODD (the first one I do not buy, but that is another story).
We have tried what you said you tried, but same as your result the only thing that happened was that he just went many days without a shower and wasn't even going to think about it.
Funny thing is that he will do so much more "work" to get out of taking a shower than it actually takes to take a shower.
For instance, when we make him take a shower he will get an attitude and spend about 20 minutes in the bathroom playing around before he even gets in the shower. Then he will get in the shower and STAND there, just outside of the water for 20 more minutes, but never wash. He always fails to get his hair completely wet to make it look like he took a shower (yep, still hasn't figured out that he would have a much better chance of getting away with it if he actually got himself wet!). He cannot make the connection that he spends 40 minutes faking a shower, when he could actually spend 10 and take one. This has been going on for years.
When I remember, I always check when he is done to see if he has done the faking - and when I do he has to get right back in. Sad thing is that if I do not stand in the bathroom while he is doing it, he will continue to fake it.
That being said, I have no real suggestions for you as we are in the same boat; however, did want to share the fact that you are not alone...and as far as our child, I chalk it up to plain laziness plain and simple. The only thing that I can do about this is what I have been doing and when he moves out of the house, he is on his own.
Maybe once he starts into puberty he will change, but I seriously doubt it.
Mike