Before you really get into the grunt work, give your walls a good going over with a duster. Forget itty-bitty rags – opt for a large, microfibre duster on a long, preferably extendable, pole. These reach all the way up to the ceiling and get to those hard-to-reach spots. Alternatively, you can use a damp cloth. If you've got a dust allergy or your walls haven't been dusted in a long time, wear a dust mask to protect your lungs.
Walls been housing a spider or two? Use a cobweb brush to get rid of their webs. These are really cheap to purchase and can be picked up at your local Bunnings.
Now your walls are dust free, use Sugar Soap wipes to give them a clean. Selley's Sugar Soap Wipes are great for smaller rooms, but if you've got a larger space you may want to consider buying sugar soap concentrate and diluting it in a bucket with some warm, clean water, and using a big, soft sponge. If you're using the wipes, like we are, start with the smooth side to give the wall a clean. If you've got any specific marks, turn your wipe over to the rough side and give your marks a good scrub.
For those particularly grained-in stains, try using a Mr Clean Eraser Pad Block. Simply cut a piece to the size you need and use it to scrub at the wall. These things are like magic, and are great for lifting pen marks or stubborn stains – just rinse with water, squeeze and off you go! Just be sure to test on an inconspicuous area if you've never used them before.