First up, the kidney can only concentrate substances up to a maximum of 1200 mosmol/L. Since we produce about 600 msomol of substances per day, that means that you have to urinate out about 500 ml per day, no matter how inconvenient this is.
Next, we have evaporation from the skin, which totals a minimum of about 400 ml per day. If you're exercising, or out in the hot sun, this amount can increase to a staggering 5 L.
Then there's evaporation from our respiratory tracts. The air we breathe in has a lot less water vapour in it than it ends up with as it descends into our lungs - water evaporates from our moist mucosa to join it. Under normal conditions, the amount of water lost in this way is about 350 ml, but this number will increase rapidly if you are breathing heavily or rapidly.
Lastly, there is fluid loss in our stools, which as we all know aren't perfectly dry. The body is actually quite good at retaining fluid from our gastrointestinal tracts, and so we only lose an average of about 100 ml per day via defaecation.
OK, so under optimal conditions, this means that we lose about 1400 ml per day, although usually it's a bit more than this. Therefore, this is the minimum amount if fluid we need to take in to keep in balance.
Fortunately, we don't have to do the calculation consciously: our intake of water is regulated by the sensation of thirst. Osmoreceptors, located in the anterior hypothalamus, are stimulated by the rise in osmolarity that corresponds to water depletion. As a result, we drink more and the status quo is preserved.
On the other hand, what if we've taken too much water on board, and need to excrete the excess? The kidneys come to the rescue here: they are capable of excreting urine with an osmolality of just 50 mosmol/L and so can get rid of large volumes of water (without necessarily increasing the renal losses of other substances). The principle determinant of renal water excretion is arginine vasopressin (AVP, also known as antidiuretic hormone - ADH). This polypeptide hormone is secreted by the posterior hypothalamus and acts on the V2 receptors of the kidney (mainly in the collecting tubules and ducts). Binding of the hormone causes these cells to insert water channels (aquaporins) into their luminal membrane, thereby massively increasing the permeability of these cells to water. The water can then passively move from the 'urine' side back into the cells and hence back to the body.
Between them, thirst and AVP are the two main mechanisms that preserve a constant body osmolality.