There are few things which you need to take care of for sure.
All blogs and talks about AMS ( Acute Mountain Sickness ) .. are true .. but trust me simple care and few breathing tips .. one should be fine and running.
The first day is very important, especially if you are flying down to Leh and not taking a road trip - your body needs to accustom to the climate with less oxygen .. I had got many pieces of advice, from drinking water to taking Diamox ... but here is what I did .. and surely it did help .
Completely Rested for first 6 hours ( recharged my camera battery, washed some TV ) - with 1 hour of sleep Took Long Slow Ujjayi ( pranayama) breaths ... Instantly started to feel better Water intake was as per thirst - not more or less Very light food - to ease digestion Took 2-3 chocolates :).. this was good energy Also there were many suggestions ranging from not to go out totally to just going to Shanti Stupa and Leh Palace .. Well I was bored and after 6-7 hours when was feeling good, I chose places where my body needs to do the least physical movement - So no Leh palace or Shanti Stupa on Day 1 -as there are many stairs and levels and the walk for the first day would have been tiring ..
So I chose Magnetic Hill, Sangam ( Confluence of Zanskar and Indus rivers ), Hall of Fame route for day 1... Best part .. you hardly need to exert at these places. Get down from the car enjoy the place, no steps, stairs etc ..
Few Pics of Confluence of Indus and Zanskar rivers