In our book, there are several ways to say thank you, here goes...
One, could just say, "Thank you."
Two, write a note. It's thoughtful, classy and memorable.
Three, you can email your thanks but only if the invite was emailed.
Four, you can pick up the phone.
Five, you can send flowers the next day after a dinner party. Try not to take flowers, the host will have to stop and find a vase, fill it with water and then find a suitable place. This precious time could have been used to mix a drink for you.
If you're dating someone, never stop saying thank you.
The same goes for spouses, siblings, parents, etc. You can never be too close or know someone too well to not thank them.
Take care, and thank you for taking the time to read this post.