Start with one room at a time. Sort through items you no longer need or use -- if you have to question it, throw it out. Consider donating items like clothes that no longer fit (or don't get worn, for any reason) to a charitable organization. Clear out kitchen cabinets, sort through piles of paperwork and throw out old cosmetics and beauty items that are past their prime.
Make the most of your storage space. Hang up clothes in closets -- consider storing seasonal items in storage bags that stash under the bed to increase space. Add drawer organizers to declutter dressers, bathroom and kitchen cabinets and desks. Group small, like items together. Store items like holiday decorations in stackable totes and label them for easy access later.
Consider adding decorative (but functional) shelving in rooms where you want to display favorite items. Add decorative baskets or bowls and fill them with items you'd like to hide away -- this works well in bathrooms with limited closet space.
Commit to staying organized. Clean out your refrigerator and cupboards before every grocery shopping trip. Toss out junk mail as you receive it -- or use the link in the Resources section to opt out of many mass mailing lists. Establish a once-weekly cleaning day -- dust shelves, toss out any paperwork and put everything in its place that may have been tossed aside during the hustle and bustle of the work week.