Not sure if this is a bug or not, but if you fail at the last step of massaging Mimi, it increases her lust. Annoyingly, the difficulty for that step is only slightly higher than the preceding steps, so it rarely triggers (<1/4 chance). I've gotten Mimi up to sexfriend status this way (doesn't do anything since there's no trigger or scene written for that, yet).
Also,
@Angus Bez could you post the save entries for the various events/sequences (like the ones for skills, stats, etc.), and some of non-obvious the tricks you can do (e.g. dating both Iris and Mayo) ? We've got everything else in the save file explained in a single post, but these are scattered amongst various posts.