A few hints on how to have a successful #PitMad:
Be sure to include hashtags for your genre and/or age group. A lot of agents filter by hashtags to find genres they represent. There’s usually a list of genre hashtags floating around, but some common ones are #PB (picture book), #MG (middle grade), #YA (young adult), #A (adult), #R (romance), #SF (sci-fi), #F (fantasy), #H (horror), and many others.
Edit: Here’s a good guide to #PitMad that includes hashtags at the bottom, as well as FAQ: https://pitchwars.org/pitmad
The best pitches are those that give the stakes: what must the character accomplish and what will happen if they do or don’t succeed.
Comparisons are also good, if they give you a visual of what the story is about: for example, Game of Thrones meets My Little Ponies give an immediate mental image of what the story might be. (That actually might be a cool book, if anyone wants to steal the idea. I’d read it.) But they aren’t necessary, so don’t force it.
To save on space in the tweet, don’t worry about including character names. Instead, tell us who they are: a poor blacksmith, a devious king, a weary traveler, etc. You can expand on that more in the query letter later.
I’m sure I’ll think of more later. I’m a veteran of numerous #PitMad, usually from the other side of the table, so I can offer help to anyone who’s wondering what agents/editors are looking for in pitches, or whether your pitch would grab attention.