I love doing touch-up painting to our white interior walls. When I'm through it feels like we've got a new home that's been bleached clean. I try to do it about every 3-4 months. Besides the smudges and streaks created by our 18-month old daughter, my husband and I leave our own "designs" on the entry hall walls and many other places.
I know every 4 months seems like a very time consuming chore but I've got it down to a science. Right now, because of my little one, I only have her nap time, during good day-light to do it. It takes me about 30 minutes total start to finish and this how.
I have my little kit in an accessible place ready to go. The kit includes: one small can of paint, small disposable roller brushes/sponges (your choice), a small roller, paint can opener and disposable plates. Paper ones done work as well as the type that look like Styrofoam but are made from potatoes, corn etc. the recyclable ones.
The plates are used to protect your work area, which in my case is the kitchen counter. One plate goes under the can of paint, another to pour the paint onto so I can roll the brush on it and not have too much quantity on me as I walk around the house. The third plate you will cut to form a lip so the paint pours out easily from the can without making a mess. (see picture)
Disposable plates and brushes are a must as clean up needs to be quick. Small brushes are also a must so you can get to the small areas between light switches and moulding where it can really get dirty. As soon as my P goes to sleep I start moving the small stuff away from walls and then go to town with the paint. I start with the area closest to the baby's room so it has plenty of time to dry before she wakes and work my way outward. Just to be safe I plan an out of the house activity for right after she wakes up so the paint really has time to dry.
I breathe so much better when I'm done. The house looks cleaner and more modern with its pristine white walls. The thing about dingy walls is that they creep up on you. Scuff marks start to become invisible as time goes on until it gets to the point where only your guests see how dingy the house.
After visiting India where EVERYONE has ridiculously dingy walls I became a little obsessed with keeping the house clean, which is how I got painting down to a science. Then I had a kid and touch-ups became a necessity.