Arm definItion can take a while to work on (and to maintain): the triceps (that "under arm flab" area) are one of the first places the body stores excess fat on the upper body.
It takes a combination of building some muscle (for shape) and reducing body fat (so that shape, a pretty sculpted shape rather than the formless blob that fat can be) can show.
Some suggestions for muscle building/toning:
Do one targeted shoulders and arms workout per week in addition to another upper body workout. Tony Horton's Shoulders and Arms (from the P90X series) is a fabulous workout, and I've heard that Cory Everson has a very effective upper body workout as well (it's available on video only, from Amazon.com).
Focus on exercises that give you more bang for the buck at first: tricep dips, lying triceps extensions, close-grip bench presses for example, rather than kickbacks, which you can add in later.
If you don't already do so, add kickboxing/boxing to your cardio workkouts. It's a great arm toner. Once you get the form down (perfectly!) you can add weighted gloves for even more toning benefits.
For body fat reduction: it's about 80% what goes in your mouth. Clean up your diet by avoiding processed foods and added oils, and stick with whole foods as much as possible, getting your fats from whole, natural sources like avocados and nuts and flax seed. Eiiminate "non-foods" (candy, doughnuts, sodas, etc.)
To aid in body fat reduction, add 1-2 interval cardio workout per week (like an IMAX, or Gin Miller's "Intense Moves" or her new "Simply Interval," which looks like a fantastic workout). Start with 1, if you aren't used to it, then add a second one (at least 2 days later) after a few weeks.