How do I know when to use avoir or être when conjugating a verb in the passé composé?

The way to remember when to use être when forming the past tense is by using the acrostic MRS VANDERTRAMP. each of the letters stand for the first letter of a verb that uses être when formed in the past tense, these are: Monter, Retourner, Sortir, Venir, Arriver, Naître, Descendre, Entrer, Rentrer, Tomber, Rester, Aller, Morir, Paritr and additionally Devenir and Revenir can be added to make it DR & MRS VANDERTRAMP. When using être as the auxiliary verb it is important to remember to make the past participle agree according to gender and number, something that isn't necessary when using avoir. For example, Elles sont allées (the additional 'e' and 's' is required) wheras elles ont mangé stays the same.