How to reduce power in barebone setup of atmega328p?
Have you considered another microcontroller? The AVR is not exactly the most low power on the market. There are XLP PICs and MSP430 too (the MSP430 is 16 bit, too).
Also you should check the power budget of your sensor; make sure you actually turn off everything you don't need while not sleeping (the ultrasound analog circuitry sucks quite a lot of milliamps in standby if not powered off); check power consumption of the radio too, maybe you forgot some standby command (I do forget, constantly!)
You power conversion setup is also quite important. What are you using to regulate power from you batteries?