Here's my tips.... never put owt that will go mouldy in your flask......milk, soup, etc. I only ever put hot water or sugared water, nothing else! I only drink tea as beverage at anytime. And I take teabags and a small stainless steel milk bottle. Hip flasks make great milk bottles.
After fishing, as you pack up, tip all contents of the flask out, dry the cup and the neck of the flask off, where the cup sits. This will if not done, create mould and taint the cup and neck. A general wash in hot soapy water doesn't remove the taint.
With Plastic bodied flasks it impregnates the plastic and is virtually imposable to remove the taint.
It's no coincidence that the food industry that deal in fresh product... milk, Milk products, meat, natural soft drinks, don't use plastic receptors on site, it Stainless steel!
If the above isn't done, fill the flask with hot saltwater, about an 1 oz of salt, leave overnight to stand, rinse in the morning with hot water and any taint has gone.
On returning home wash out both flask and milk bottle with hot water, and you never have the problem of mould or tainting.
