Does your camera go into mini-sleep mode after some seconds? In that mode you have to touch shutter button to wakeup camera (which behavior is actually annoying, but cannot be turned off). (Using IR remote it doesn't.) If your camera doesn't enter into mini-sleep mode, then something is hindering that - I would suspect OS. My only OS lens (18-200) for example caused camera to not able to turn off at all, while in sleep mode.
Do you use continous AF mode? (I don't)
How long are you set live preview time? (Mine is 2sec)
What is LCD contrast and brightness? (Low)
How often you turn LCD on for review? (Rarely)
Have you focus confirmation beep enabled? (No)
What firmware have you installed? (1.05)
Are you using flash? (No)
What is your usual shooting temperature? If below zero (C) for example, then battery capacity is much less than at room temperature
Does charger used (you should have two) make any difference in battery life?
Can you check battery life, using non-OS lens and MF mode, without live preview (and without flash)? This should give more than 18 images for sure (and makes you a better photographer
). If it doesn't help, then something is wrong with your camera(s) or battery(s).
What about different kind of batteries, to my Sigma original lasts much more than cheapest clones (which in addition tend to discharge over time, even not in use - and one of these doesn't work anymore after 3-4 recharging cycles). I've read that best would be Pentax D-LI50 orignal battery, it has usually even bigger capacity than Sigma one (otherwise they are identical). And find some better charger