42mm owners are going to have to do a bit more maintenance and power saving to get a good day out of the device. Here are some tips:
Use a predominantly black watch face
Turn AOD off (I know, it's a shame, but thems the breaks)
Set screen timeout to 10 seconds
Get into the habit of shutting the display off rather than waiting for the 10 seconds: use palm off or flick wrist away if you have wake up gesture turned on
Keep plenty of storage free (at least 1GB)
Get into the habit of closing running apps
Clear cache and RAM frequently
Get rid of apps you don't use
Change HR monitoring to frequent
Turn WiFi completely off when you don't need it
When you can, turn brightness right down to 1 or 2.
If you are really running low on juice, turn on power saver.
I'm not saying you'll like these things, but they will help.
Also recent/ latest update also claims improvement in battery life.