First, be gentle when you talk with your spouse on this issue. Find the right time and do not speak in anger or frustration.  When you talk, tell them what you find attractive about them beyond the physical. Praise them as a mate and friend.

One way to lead into this type of conversation is to share with your spouse your desire to improve yourself. Ask them to share with you how you can better serve them. Remember, marriage is about serving and each of us should desire to improve our serve. When you share opportunities for improvement, be sure to let them know you want them to be their best, you do not want them to be somebody else. 

When you share ways they could present themselves more attractively, be specific. Examples include: a haircut, shaving, diet, exercise, clothing and bathing.

Be willing to sacrifice to help. For example, many people know they need to exercise but they don't have the discipline to keep a regular exercise routine. As their spouse, be willing to joining them at the gym and support them each step of the way. If they are considering weight watchers or a special diet, be willing to join with them to be their support and encouragement along the way. 

Our sexuality is a God given gift that enables us to serve our spouse in a way no one else can serve. Taking good care of ourselves is striving to serve our spouse in a way no one else should.

It shall not be so among you. But whoever would be great among you must be your servant, Matthew 20:26 (ESV)

It is also important to remember that God views us, and will ultimately redeem us, as a whole person, both body and soul. Just as we spend time in God's Word to nourish our spirit and draw near to Christ, it is important to care for our body which is the temple of the Holy Spirit.

Or do you not know that your body is a temple of the Holy Spirit within you, whom you have from God? You are not your own, for you were bought with a price. So glorify God in your body. 1 Corinthians 6:19–20 (ESV)