Well, from someone who are not residing here for that long, yet has "some" experiance in RP...
First thing I do - I look for inspirations. As it was said earlier, movies, TV-shows, games, books, even pictures on Pinterest, all that can feed my imagination and pull some strings. Though, to be honest, it is hard to pull off some good characters and RP as them without any inspiration, it is doable. But why setting it on hard-mode for youself? Use any source for that spark that you can find and in the end - you won't regret looking. What also comes to that part - I got used to choose a music theme for my characters, which really fits their overall mood and concept.
The other thing is - try to write up your characters background. Even if you are able to hold it inside your head and improvise, write it down. It really helps both in stabilizing your thoughts on where your character comes from, what they lived through, and in getting immersed into your char. At times, you will want to add details, live tthe story you are writing. That what helps me the most. Not mentioning being more confident in what you are actually playing.
The other thing that helps me - is like Jazzi said, finding some details and, well, quirks that you can emplement into your gameplay. Even adding some words from character's "native" language works, especially if you do know why aren't they speaking clear English. Works perfectly when playing someone from Gallia, for example, considering them being isolated from Sirius for quite some time.
