Hi/Lo is all about maintaining interest of someone older while keeping the reading level in mind and making sure you aren't talking down to the reader. So one tip is to look at your readership and know what will interest them. Another is to look at catalogues of publishers doing these books and the books themselves. Saddleback is one such publisher. If you're pitching. you'd want to find something they don't already have or fit a series they have with a new topic.
Consider age and grade level also. Hi/lo for a fifth grader may be at a second grade level, whereas hi/lo for a high schooler could be at a fifth grade level. And be culturally conscious. Recent publishing pros I've spoken to have been very much into sensitivity and diversity in all of their works.
Sorry I don't have links to offer.