I know exactly what you mean. I came from Yahoo! Chat roleplay, which is where I spent a large majority of my RP life. There, I used primarily "para" style roleplay, short for paragraph - which, as the name suggests, involves about a paragraph or more per post. I did very little forum roleplay before coming to Iwaku.
My suggestions to you on adjusting are:
- Where it's reasonable, be a little bit more descriptive. Not by much (we don't want you to wander into purple prose territory), but set the scene, explain your characters' body language, paint a backdrop to allow your readers to understand the image you're painting for them.
- Move the plot. Chat roleplays tend to have a lot of back-and-forth banter, with stories that kind of arise organically from the interactions between characters. While this is all well and good, MOST forum roleplays (not all) have an over-arching plot or theme. If you know what direction the story needs to go in order to progress, try to be proactive by leading it there rather than only reacting to previous posts.
- Write scenes, not lines. In chat roleplay, each post is typically only a few lines of dialogue. In forum roleplay, you have a little more freedom to lengthen them into a full scene or a mini-scene. Imagine it like a brief scene in a movie or a TV show that focuses on your characters and their interactions, before cutting away to the next scene involving other characters (i.e. someone else's post).
- Collaborate! The important thing to remember is that roleplay is collaborative writing. Chat roleplay tends to be very spontaneous - this has its own perks, but also has the disadvantage of lack of planning beforehand for whatever is about to occur. In a forum roleplay, you have the time to stop, message your roleplay partner(s), and discuss what needs to happen or share ideas before proceeding with your post.
- Have fun, and don't be afraid to ask for help! Iwaku is an awesome community with lots of people willing to lend you a hand. Don't take yourself too seriously and remember that we're all trying to tell a story together. ^^