What are the precautions when using a life jacket?

"Life jacket" only refers to a very generic term, Personal Flotation Device (PFD) is more acceptable term that covers a larger group of devices.

Ensure you have the proper size of PFD. Ensure you are wearing the right type ( i.e. type I, II, III and so on) for the right activity is also essential. Have extra PFD's easily accessible and not locked away and last but not least have everyone wear one. Cool or not, PFD's are documented lifesavers. See the link below.