A great deal of what looks like low desire is actually not wanting to be looked at. The two are easy to confuse from the outside and feel completely different from the inside.
Say what would help
Your partner cannot guess whether you want the light off or want to be told you look good. Most people guess wrong and then stop trying.
Specific beats general
"You are beautiful" glances off someone who does not believe it. Something specific and true is much harder to dismiss.
Watch the throwaway comment
One careless remark about a body can be remembered for a decade. This is one of the few areas where a single sentence really does do lasting damage.
Confidence is contagious in both directions
Someone who is comfortable in their own skin makes it easier for their partner to be. The reverse is also true, quietly.