I do relaxation exercises with Charlotte. She closes her eyes, I stroke her head and we concentrate on breathing, relaxing and calming her body, from her toes to the top of her head. Monotone stuff, each breath out, a part of your body relaxes further, becomes lighter and calmer....
9/10 the "I'm not tired" 5 year old is asleep before I get to her shoulders.

If she really isn't tired, we sometimes find actual physical exertion helps - we used to have a large island bench, and exH would chase her around the bench, she thought it was all hilarious, but after 20 minutes, she would be zonked. See how many lap-lengths of the house she could do, star jumps... then do the bedtime routine, and she would be out to it.

Do you read to hear at bedtime? Both of mine will fall asleep if I sit there long enough - about 30 mins of reading a non-picture book works well.