Okay I have this problem, I admit it. When I step into the shower in the morning, and that deliciously warm and soothing water washes over me, it eats my brain and I turn into a shower zombie. Five minutes, ten minutes, fifteen minutes… I'll just stand there, slowly rotating until the water gets cold.
It's just so relaxing and comfortable, especially when you are sore from activity, that it numbs me into a near-catatonic state. I say near-catatonic because I am moving around and thoroughly soaking myself, and because I am quite actively thinking. I get some of my best thinking done in the shower. I think of everything except how much water I'm wasting.
My wife and I shower on alternating days, and my kid showers or bathes in the evening, so it's not like I'm keeping anyone out of the shower, but I have made us late for engagements before. No matter how I try to keep in mind that I'm in a hurry, the shower washes my urgency away.
For years I've tried to figure out a way to combat the brain-washing effect of the shower. It's not like I want to waste water or turn into a pink raisin. I tried putting a clock in there. No good, I just stop looking at it. A timer is no good either because I will end up getting out of the shower, dripping everywhere, to shut it off. Then I just get back into the shower and rezombify. Then a few months ago, I finally hit upon the solution.
Count.
Yep, that's all there is to it. Counting is a very simple operation that is hard to screw up, and I simply focus on the fact that I cannot stop counting until I get out of the shower. I count seconds, and try to be out of the shower by the time I reach 300, which would be 5 minutes.
By 60 initial wet-down must be complete. By 120 my hair should be all shampooed up. By 180 lathering should be done. Rising off the soap is finished by 225 and rinsing out the shampoo is finished by 270, which leaves me thirty seconds to wash my face and turn lazy circles until 300.
Even though counting is boring and I may stop every now and then unintentionally, I can just add 20 or 30 seconds to wherever I was and keep going. Once I reach 300, if I'm not done, I jut keep counting and give myself a short extension, to say 360 or 420 (a six or seven minute shower.)
That's the beauty of it, even when I'm “late” I'm usually only late by a minute or two, and the counting keeps me focussed on getting clean instead of just drifting off into la-la land. So now I only take a long shower if I want to.
It's not a perfect system of course, sometimes I'll get hypnotized and realize some minutes later that I stopped counting, at which point I may just say “screw it” and finish up the shower at a leisurely pace.
But overall my lengthy showers have become much shorter, and will remain that way. You can count on that. 
Why not just get in the shower earlier and take a long shower every time. If you enjoy it that much, do it up!
Of course, there ar ehte water and heating bills…
I don't have the same problem you do, but most watches nowadays are waterproof. When I see that I'm going to be late, that usually spurs me to hurry up in the shower.
The real issue, however, is the thinking. I do a lot of thinking in the shower as well, but sometimes I forget my ideas. I need some sort of notepad in the shower. Better still would be a waterproof voice recorder where I could download the resordings to my computer as MP3s.
Heh. I can just imagine you trying to listen to those MP3s later…
“SSSHHHSHSHshhshshshwhwshshhshshsh -reat idea- wisssssshh splorsh -at if instead of- SPLOSH -ing with utensils one were to- SHHSHSHSHshhs PLISHsplishshhhhhhh”
In the words of Dr. Lizardo:
“Laugh while you can, monkey boy!”