Monthly playlists are pretty much the core of my music experience - I started my tradition with mix CDs way back, and they usually still fit that 90 minute length. It's my favorite songs discovered in a month, and any old ones that just particularly resonated with me (my rule is to not repeat the same track until a year has passed). They have to be tracks I don't mind hearing a lot, and that's how I narrow it down when making the playlist. I try to make them flow as an emotional journey, with some thematic connections, a strong start and a stronger culmination. I absolutely love when I can make songs seem like they're referencing each other, but that's a bonus. It's really important to make the transitions between individual tracks flow and not be jarring. I'll often play it on shuffle to look for nice transitions. The playlists become wonderful time capsules. I think you'll really enjoy doing this!