Hi and welcome to the forum!
The SIIX is a bit of a pig on the battery, mosty due to the large screen, but also due to the 1500MHz CPU.
EDIT: Using the power saving feature that is built in won't do much until the battery reaches the critical level set in it. You can turn GPS, Data and WiFi off when not used, but manual manipulation is a pain after a while. Option 1 below solves this issue. /EDIT
Your best bet to saving power are the following...
1) Use Juice Defender (JD for short). I used the free version for a few months and loved it. Once I was comfortable with the features I paid for the Ultimate version and created a power profile that suited my needs. My battery goes for more than one day with regular use and I can get 2 days out of it with light use.
2) Screen brightness - Set it to automatic. If you use JD then you can customize the auto feature.
3) Autosync - There are several apps that autosync and consume power by using Data or WiFi. Change the autosync settings, or turn them off completely. I don't autosync anything. Go to Settings -> Accounts and sync
4) E-Mail - It is an item of debate, some say pulling in e-mail every 15 minutes eats battery, others don't. I have 3 POP accounts that are set to pull the full e-mail in every 15 minutes. Since I don't autosync other items, this seems to have no ill effect on my battery.
5) Apps (from Google Play) - If you have tons of apps installed and some that autorun upon boot of the phone they take up RAM and CPU cycles. More RAM and CPU use drains battery. If you don't really use the apps, then I suggest uninstalling them. You can see what runs automatically by turning off your phone, booting up (completely) then go to Settings -> Applications -> Running Services and see what is in the list. If there are apps that you have downloaded from Google Play in the list and you don't really use them then get rid of them.
That is the short list. There are many, many suggestions on battery saving, but these should really do the trick for you, specifically Juice Defender.
