I use boost up converters like the ZXSC100 in high volume. We boost 3 volt batteries up to 5 volts. There are many different types of boost up ICs Some of them work down to 0.8 volts input.
http://www.zetex.com/3.0/pdf/ZXSC100.pdf
http://www.zetex.com/3.0/pdf/zxsc410-420.pdf