Backstage always has some good materials, if sometimes more for those just starting out. That being said, it’s always important to remember the fundamentals- a good monologue can really reinforce that. This article touches on some things to look out for when choosing a monologue, but it also misses two key things: connectivity and honesty.
Connectivity is being able to really resonate with what is going on in the scene and to the character. For example, as a man, it is going to be harder for me to establish myself in a monologue that is written for a woman. Not impossible, mind you, just more difficult. Honesty, of course, is the lifeblood of great acting. It is being able to find the truth in the scene by adding yourself to it. Not a shot at method acting, which is simply just a different way of finding and displaying said truth.
That being said, it is always better to choose a monologue that you like, and then work on that piece til you can do it in your sleep. Just don’t let it become robotic and stale!