I am using a app called Kpowersave.
When you are doing basic tasks, such as web browsing and text editing, you are not using the full potential of your CPU, but it does consume as much power.
Kpowersave clocks the CPU back when you aren't using any demanding apps, and instantly clocks it to maximum when you need all of your CPU speed, thus saving power.
If you play 3D games a lot on Mint, this may not be a solution that helps a lot since you will constantly need all of your CPU speed, so your CPU will be consuming maximum power most of the time.
Kpowersave extends my battery life with an estimate of 30 minutes to 45 minutes.
It is strange that Mint depletes your battery faster than Vista, you would expect it to be the other way round.