Encouragement is fine, helping is good, but odds are, they've been teased about this for a long time. Megastores can help. They have a wide assortment of organizationy things and for fairly reasonable prices. This will not be solved overnight. If you're living with someone who's messy, or loving them without the benefit of cohabitation, this will not be something that changes over a day or a week.
Patience is really important.
Remember, you love someone for who they are, not because they're neat. If all else fails, bring in professionals. You may need to consult with an expert. Don't give up hope. There's more to life than cleaning.