Lots of theme songs. Every single one of my characters has a Spotify and/or Youtube playlist that I play in the background whenever I write about/for them. Couples/pairings and certain friend or family groups sometimes also have their own playlists dedicated to them as well depending on the dynamic and whether I could find enough songs to make a playlist out of.

The idea is that the songs in the playlist always remind me of certain parts of their history/biography (often at different parts of their life, some songs will refer to their early history while others refer to more current history and so on), their personalities and motivations, and thus I am motivated and remember [i]why[/i] they are who they are, as I'm writing about them. It's worked pretty well for me so far :) 90% of my characters started off as a really simple one sentence or paragraph concept describing the basic idea of them and then became more fleshed out as a result of a combination of developing their playlist and just "winging it" in some roleplays as they go along :)

Alternatively, re-reading previous posts/scenes from the RP also helps because I can then see how they used to be at different portions of their life and compare how much they have changed/grown as a result of the people they have interacted with throughout the course of the RP. 

And as cliche as it may seem, sometimes "just rolling with it" and writing whatever pops into my head works too. Sometimes the characters run away with me and decide what they want to do all by themselves (which, granted, sometimes [i]also[/i] creates [i]more[/i] problems than it solves! :lol particularly with certain characters ;) )