[Samsung Galaxy W I8150] egagah:
when you just flashed the battery will definitely be shorter. Let it recharge for a few cycles before determining whether its good or bad battery life. Keep in mind however that even at full potential, the battery will not last for more than 2 days even at light usage.

if battery life is more important to you compared to "the newest OS" and whatever extra features it brings with it, then I highly recommend installing acro's CM9 RC2 (yes it is ICS) as the 2.x.x kernel is far better at battery life than 3.x.x (i'm currently testing it and I can already feel the difference! I believe I can reach 2.5 days at modest usage but I will have to update you on that after my first few discharge cycle for battery to reach potential after flashing new rom). I do, however notice a few annoyances with RC2 (not sure if its really the ROM, or the custom kernel i'm using, or Nova Launcher is to be blamed), so if it continues to bug me I may just switch back out to latest ICS =p.

If keeping with the latest JB is important for u, best thing you could do is download a custom kernel for it that supports UV(undervolting), then get IncrediControl and tune down your UV based on some voltage tables you can find in FAQ. This alone won't bring you a HUGE improvement in battery life but its definitely an observable difference. On top of UV, try to UC(underclock) your CPU to 1GHz. Most daily functions, apps and games (depending on how intensive it is) will work perfectly smooth even at 1GHz. IMO, the most important point of UCing is you will notice a very significant difference in the heat of your battery (ie backcover of your phone). Heat is generally bad for the battery (shorter life + faster draining) so UC is definitely a good thing to do =). Of course, on top of all those, get greenify and wakelock detector(WLD) from market to control your not-so-frequently-used apps. Use WLD to track down what is keeping your phone awake (hence drain more battery) and use greenify to hibernate all the not-so-frequently-used apps you have (example of things you DON'T want to greenify: whatsapp and gmail as you don't want to kill these 2 programs when you lock your screen. You want to continue receiving whatsapp messages and gmail when you locked your screen right? XD! Examples of things you SHOULD greenify: games, music player. Maybe some of the games or other apps may keep a background service active, secretly using your battery even though the last time you used it was 2 days ago!)

Finally, if your wonder is about 1-1.5 years old, chances are your battery is already bloated (perhaps you can feel your battery through the backcover of your phone even!) A bloated battery is a dangerous and short-capacity battery. Even with the best rom you may only push through a day from 100-0%. If your battery still looks fine but your phone is nearing its birthday, go ahead and do some online shopping (ebay or local equivalent, like malaysia we have Lelong.com.my) and get a battery for your phone. Its usually much cheaper than buying from telephone shops.

EDIT: Adding on to all of that, choosing a good governor+i/o scheduler combo is important as well. If our phone is similar with the S+, smartassv2+sio(i've used this, its quite smooth and battery is good) OR OnDemand+noop is thought to be very good in terms of battery life and responsiveness. Lulzactive is also very good but its not so easy to tune it so i usually would stay away from it. Do bear in mind that although the former 2 combo is good, it still highly depends on how you use your phone so there will not be a universally best governor+io combo. Both of those suggested by me are supposingly best in responsiveness as well as deepsleep battery conserving (only if the assumption that our roms work the same as the roms tested in S+, which MAY NOT be true).

TL;DR? try smartassv2+sio (u may change this using IncrediControl, get from playstore). Then if you are not satisfied with your current performance/battery life, you may continue experimenting by changing governor and i/o scheduler one at a time. If you are doing the testing, make sure to have some way to record your tests if not you will 100% be deceived by placebo!(with actual numbers you will clearly see whether your feeling is placebo or if there really is an improvement)