When do I use être when forming the perfect tense?

It can seem confusing when deciding whether to use avoir or être to form the perfect tense. As a rule of thumb the conjugated form of avoir is used except with:
a)Reflexive verbs.Eg: se lever - to get up je me suis levé(e) - I got up
b) Verbs denoting a change of state.These can easily be remembered with the mnemonic DR & MRS VANDERTRAMPDevenir (to become)Revenir (to come back)&Monter (to go up)Retourner (to return)Sortir (to go out)Venir (to come)Aller (to go)Naître (to be born)Descendre (to go down)Entrer (to enter)Rentrer (to go home/to return)Tomber (to fall)Rester (to remain)Arriver (to arrive)Mourir (to die)Partir (to leave)
When using verbs which take être it is important to remember that they need to agree with their subject. Therefore an 'e' is added if the subject is feminine, an 's' is added if the subject is plural and 'es' is added if the subject is both feminine and plural.