Getting the most battery life out of an Inspirion 9100

So yeah, I kind of screwed up in getting this workhorse. It's a good laptop no doubt, but when on battery it's not quite the champ. The model I got is:

P4 2.8 HT
512 dual ddr
9700 pro
60gb hdd
dvd drive

Now, on my battery I can get 1:30 out of it on a good day, and God knows I need more than that. For those of you with a similar laptop (9100 or XPS), on average how much life does a secondary battery add? Is it worth the price in other words.

Also, are there any third party battery packs (not the ones that go in the laptop, but rather external) that you guys recommend? Weight would be an issue, it can't be too heavy as my laptop is already pushing 8.5lbs.

This laptop is needed for school, as I'm headed out to texas tech in about 2 months, and have a limited budget. Please let me know, thanks!