Step #1: The Clutter-Free Zone. Set an area of your house to be the clutter free place—nothing that doesn't belong is allowed! Keep this place clean no matter what; get yourself into the habit of cleaning to help with the rest of the house.
Step #2: Pick an Area to Declutter. Where is that one spot you're dreading? You know you have to clean it up, but you really don't want to—start there. If you get the hardest out of the way first, you're likely to feel a boost from finishing it and get a flood of motivation!
Good places to start decluttering include desks, tables, corners, coat closets, bedroom closets...
Step #3: Work Around the House
This is the best idea and most important principle yet: take the places you de-clutter and use them as motivation to finish. It's also important to maintain the places you've already de-cluttered, because it can build up fast.
Over time, whenever you have 20 or 30 minutes here or there, find a new area and declutter it. You'll be amazed at what just a little bit a day can do over time!