Some useful tips I have developed over my teaching career is to exercise the diaphragm muscles. For example, descend a scale using the diaphragm only; i.e. without tonguing, by producing a laughing sound between each note, then sustaining the note with a regular pulse eg. Huh, Huh, Huh, Huh Huuuuh. This is quite exhausting, but after some time you will be able to develop a natural vibrato and an amazing sound.