- Improving your vocabulary to an extent could help, though it's not vital, it'll just help you express yourself better.
- Draw inspiration from people/yourself, not in an RP sense but more so in a sense of if you're RP'ing a specific type of person you usually have a pretty good example of them in your life or in popular media, that and just visualizing them interacting with whatever situation you're in assist you to maintain a lifelike feeling to other people you interact with, or even try and visualize yourself in said situation.
- Always keep in mind the emotion of whatever situation you're in, if you're being beaten for example, you obviously experience pain but it's obviously not the only emotion you're going to experience, things like fear, hesitation and alike follow suit.
- Have a very clear idea of what your character's like, their behavioral patterns, their motives, ideology and alike, though despite that always keep them grounded in reality, if for example a trait they possess is fearlessness or a lack of seriousness, it wouldn't exactly make sense if they keep that up when threatened by Civil Protection or at gunpoint.
- I always make a character by considering their personality, then everything else follows suit, as I think more of how the character is I consider on how to make their backstory, for example if they're a good hand to hand combatant it might be due to them studying a martial art when they were a child or due to them growing up in a rough environment that demanded that of them.
These are just things that came to mind off the top of my head, everything else is mainly due to experience and whatnot.
Just an important thing to remember volume=/= quality, I often see people typing monumental /mes but I see several full stops ('.') in their /mes or just pointless filler, odds are that if it got to that point that /me would've done better as several /mes, just because a /me is long doesn't mean it's good, contrarily, it normally means the opposite.