Ask them how they feel about the relationship first. You may find that they've been feeling the same way and your relationship is coming to a natural end. If so, you can agree to part as friends.
If your partner is still in love with you, I agree with 'keepitnatural' - be honest about how you feel. It's tempting to sugar coat, but try to avoid fudging your words. It might make you feel better in the short term, but it's more respectful to make a clean break.