How to clean inside porcelain teapot?

Swish some ice cubes with cut up lemons around in them, then wash and rinse.

I always use a spoonful of clothes washing powder in the teapot. Fill with hot water then leave to soak until the stains come away. Make sure you rinse the teapot thoroughly before using again!

Of what?

I would caution that vinegar is an acid and could pit your silver teapot, especially if it is silverplated. I've used vinegar with porcelain teacups and pots but only if it really needed it cause...

I use bleach to clean the insides of things I can't reach into. Let it sit a few hours, it works great, unless the decanter is made of something that you can't use bleach on.