Backlight - turn down the intensity and the duration. Note there are separate settings for During Activity, and Not During Activity (ie. 24/7 watch mode)
Some custom watch faces are also big power drains.
So check your settings for all of these.
Also, after any update (and often the watch needs an update when you first receive it), it is a good idea to power down the watch, and power it up again. This can often reduce battery drain.
