Turn WIFI off, use 2G only and of course as the screen uses the most juice, turn the brightness down to about 15%. You can find more tips by search the forums. Also check if anything other than screen uses more than 10% (with medium usage).
I can get 3 days with my DHD, but that's with minimal/ no usage.
2 days with some usage and 1 day with heavy usage.
I usually end up with about 20% left at the end of the day