Reading books and reviews – knowing your collection – is very the best way to develop your readers advisory skills. Publishers webinars and catalogs can also be helpful.
For children
RA for Children: How-To
Five Finger Rule
Book and media lists from the Association for Library Service to Children (ALSC)
Notable Books and Media for Children from ALSC
For tweens and teens
Book and media lists from Young Adult Library Services Association
Flowcharts
For both
Find the next book in a series
Booklist Webinars (free)
Passive RA