How do you make your voice smooth after singing for a long time?!

How do you make your voice smooth after singing for a long time?

When I sing for a long time, my voice gets tired and a little sore. I heard that drinking honey/lemon helps, but I tried it and it doesn't work. I think that's for a sore throat that's cause by like, the cold or something. This is different. When I first start singing for the day, I can go into falsetto from chest really easily and smoothly, but after a while of singing, it gets harder and the switch from chest to head gets really jumpy.

I noticed brushing my teeth vigorously helps a little bit, some how. I think it has to do with saliva, but I'm not sure. Drinking lots of water actually makes the problem worse, because it makes my throat a little more dry. Any suggestions?

Please, no opinions like "you should be happy with your voice" or "I dont know, try some honey in tea". If you dont know, please dont respond. I'm looking for a real, 'scientific' answer.

I've tried :
Honey/Lemon.
Honey in Tea.
Lemon in Tea.
Tea.
Ginger.
Lots of water.

None of them helped.

sounds like overwork coupled with some stress/strain in your throat muscles.
How long is "when I sing for a long time"? 1 hour or 4? If you must sing for more than 2 hours in a row, you will either have to accept vocal fatigue as a reality of life, or do something quite shocking- take a break in between for 20 min, and absolutely shut up.
Drinking "lots" of water is also not right. Sips of room temperature water are good, helping rehydrate the tissues. Too much water, and your body will want to gt rid of it, also pulling more water out of those same tissues in an attempt to relieve the build-up. Unless you are under klieg lights, sweating up a storm, you don't need more than a few sips at a go.
( I work in a middle-sized opera house in Aachen, Germany.
Rehearsals run for 4 hours, with a 20 min. break. Most of us will sit and have a drink in the break, and keep fairly quiet, unless there is some organizational work to be accomplished, which happens fairly often, but we try and rest our voices during that time. Performances are actually easier in that regard- you need to do the scene only once, and generally you are off until your next scene. Time enough for a quick sip and a rest. No one would ever think of 4 hours without a break.)
When you have finished your singing for the day ( or night)
don't forget about cooling down. It's about as important as warming up. Hum, sigh, stretch your neck and shoulders out, do a last couple of lip trills or tongue trills, and make sure you are releasing any built up tension.
I would invest the time energy and money and go to a coach or singing teacher to have another set of ears help evaluate whether or not there are any flaws in your technique that have crept in. ( there usually are!) Too much strain in your throat musculature will show up exactly as you stated: tired and sore. If the symptoms go away, but not the cause, you are looking at a very short-lived singing career, before you do yourself damage. Be kind to yourself and have another opinion. It may prove detrimental to your ego, but that's easier to repair than vocal damage, any day.
Best wishes and keep on singing