Baseboards can be a backbreaking job, but with proper, and frequent maintenance, you can limit the times you have to get down on your hands and knees to once or twice per year.
First, vacuum them with the small brush attachment. If it is still dirty, then this is where you must get down and do the tough work. I spray with a cleaner like Formula 409. Don't over-wet, though, or you can damage your drywall. Let sit for a few minutes. Then I use a Magic Eraser and a toothbrush to get in the hard to reach crevices. Don't scrub too vigorously or you can damage the paint. Let dry. Beautiful, huh? Yep, but HARD work.
Here's a VERY IMPORTANT last step that can keep your baseboards clean longer with only a light dusting. Fabric softener sheets! It really works! After the baseboards are dry, rub a fabric softener sheet along them. The dirt, dust, and pet hair are repelled from the surfaces. Oh, and the fabric softener sheets also works on window blinds.