I commute 11 miles to work, and at least once a month, at about mile 2, I'll look down and notice I left my water bottle on the counter at home!
What tricks do you have to make sure this doesn't happen to you?
I'm not a commuter, but I believe in putting something that must not be forgotten next to something that might be forgotten.
For instance, I would put my bike gloves on top of my water bottles.
(I do the same thing when I have people over to visit. If there's something in the fridge that I want them to take home, I put their car keys in the fridge on top of the container.)
I hang my helmet on the bottle cage. I almost always notice it's empty when I grab the helmet.
In the past I've stuck a sock monkey in the cage at the end of the ride. I thought I would always notice the sock monkey. But when you start out early in the morning sometimes you don't see things like that. I finally figured it out from the funny looks I was getting. So while thirsty, I still brought a smile to everyone on the route that morning.
You might get into the habit of inspecting your bike before you take off. I check the tires and the brakes every time I pull the bike off its rack. Check your cages along with it.
Mentally I classify water along with protective gear, probably giving it more weight than a helmet. It isn't something that's merely nice to have because you're a little thirsty. It's something that stops me from having a stroke in the heat (and cramps when I'm done).
Vaguely related. I keep $2 on my bike so I can buy a bottle of water if I have to.
You could also place something in your cage that won't let you operate the bike after you get off it when you're done. It's easy to remember your water bottle at the end, because you're thirsty. Pick up the water bottle and place your pump or something in the cage so you have to pull it out and remember the water before you leave next time.
My advice though is to treat water with the importance is has.
i have developed habit to drink just before riding (always). even if not thirsty which mean i'll have to check my bottle, and if something wrong (like the bottle was empty) i still at home (office)
If I forget to refill, I have the other water bottle (and probably half of the first). I also use decent quality water bottles that don't make the water taste too bad if it's sat in the bottle for a few days.
If for some reason I'm leaving the water bottles inside the house, I'll put them where they block access to something I can't leave the house without, such as putting one in each shoe. (no, really)
* 3 bottle cages, actually, but one holds a travel mug that sometimes gets a hot beverage
I carry my water bottle(s) with me if I'm going to be somewhere without convenient (or cheap) access to decent beverages for an extended period of time. This includes classes, driving for more than a few minutes in the car, movie theaters and fast food restaurants that charge $4 for a PET water bottle, etc.
I have been doing this regularly for about two years now, so anytime I leave home without a water bottle, I can't help but notice I'm forgetting something.
I also commute 20 miles in a day. I have ran into your problem more than once. However I have a small trick to stay on safe side.
I have a back bag, which has separate pockets for water bottle on two sides. I have two bottles, One I always keep on the bag, and never take it off unless I am refilling my actual water bottle. Another I keep at the bike.
Whenever I get off the bike, i put that bottle on my bag, and when i am riding again, i put in back on the bike. This way, even if forget about the bottle, it will remain in my backpack, which I hardly forget.
In case I forget to refill, I will have a extra bottle, from which I can refill the bike. Or in the worst case, if i left/lost my bike's bottle somewhere I already have a bottle at my bag, which can handle the rest.