Don't compare yourself to everyone else
Whether it's the number of characters you have, the type of plots you're in, how long your posts are or how "good" they are, just stop it.
Constantly comparing yourself to other people will make you too paranoid to actually RP with them. It will also bore you. Then you'll leave. We don't like that!
Communicate!
If you're not sure where a thread is heading or if the other people are going to be comfortable with where you're going to take the thread, PM them. It exists for a reason.
Take the initiative
Say you made a thread in the RP requests forum. It's up to you to start the threads people offer to do with you out of that request.
Not only is it polite, but those other people? They already have plots. They probably have stuff they need to do in real life, too. If you want to get your character active, you have to be the one doing something about it.
Don't Plan
Or, if you do plan, don't expect things to go the way you planned. You're playing with people who have their own ideas and initiatives. There's no way this is going to turn out how you expected. And lots of times, the ideas you never even thought of are way more interesting and fun.
Make some assumptions
Because the wizarding world is so small, if your character is in a thread with someone a similar age and went to Hogwarts, you can save a lot of time by assuming they at least know who the other person is, even if they don't know them that well.
Embrace the awkward
Roleplay is basically about telling a story and stories are basically about people being pushed out of their comfort zones.
Don't try to avoid the awkward moments in threads. Embrace them. Amplify them.
Fear is fun
Figure out what your character fears most and aim it at them. Your character development will thank me.
Prepare for your character to die
If you're okay with your character dying, you are way less invested in them being safe all the time. You're more likely to embrace the consequences of their actions when they're in risky situations and to be more open to alternate ideas from other people.
Act out the RP
You will feel like an idiot, and you may have to lock yourself in your room, but if you're having trouble with the logistics of what your character is doing, or wondering if your dialogue sounds realistic, act it out.