Water is to humans as oil is to vehicles.... Except more.
The more often you change it out, and the more you have in your system, generally the better. It's very difficult(but possible) to consume too much water.
As for losing weight, no it does not directly affect it. However, it is a VERY valuable supplement in conjunction with other health plans. Besides keeping the body in better functionality, if you can train yourself to drink water when you're thirsty(instead of juice or soda) you will be drastically reducing your complex-sugar intake. That will do a great deal to fight weight gain, as beverages are often some of the most sugary stuff you can ingest.
Water consumption is something I'm very good at. I've all but eliminated most beverage consumption. It's a habit I've been building over the past 2 years. I believe my success in this area is due primarily to three things:
1) Get a filter/pitcher and keep it in your room next to your bed or computer along with a glass. Always have your water immediately convenient.
2) Get a good, big, durable, convenient water bottle, and take it with you when you go places. Sometimes it's not realistic to carry it around, and that's alright. But the big idea is to try to increase it's availability.
3) "Hi, can I start your order out with a drink?"
"No, water's fine for me, thanks. For an appetizer, I'll have....."
One more idea that a lot of people suggest: put a lemon slice in your water bottle or pitcher, or perhaps a little bit juice powder mix for a bit of taste. Yes, it adds sugar, but it's a better alternative than drinking soda and juice.