Hi, D..
I have three children ages 15, 12 and 8. My oldest has always been a little scattered, but my middle child has always been very responsible. In my experience around the middle school grades, even responsible children become rather forgetful. . . homework never makes it to school, when leaving one room to travel to another in the house, they don't remember why. . .
Be consistent with your expectations. Encourage your daughter to write things down. We bought a dry erase board and a day planner for my oldest to help her stay focused. Consider developing a "contract" that outlines your expectations as well as rewards for keeping up with the chores or consequences if she doesn't. There are some great samples online. Contracts are helpful when she gets a cell phone. . . or a myspace page. . .or begins dating or driving as well. Putting your expectations in writing and making her sign it takes away the "I didn't know" argument. The key is to let her be a part of the decisions. . . she might be able to suggest things that would help her remember more often.
Also, help her set up a schedule to manage her time. If she is watching TV, talking on a phone, texting, chatting on the computer or even listenign to music when she needs to be working, it can be distracting--making the chores take much longer than necessary. Require that she "unplug" for a certain period of time each day and use that time for chores, etc.
I also have a friend who leaves "funny" sticky notes out for her family. . . for example, if the trash needs emptying. . . she might leave a note on the can that reads: "I smell something funny. Would you mind emptying me?" Or "We're being invaded by furry creatures from another planent. . . oh no, it's just dust bunnies. Please dust today." This doesn't work for my family, but she's had great success.
Good luck!