Generally I make very long Rp posts when there is a dramatic or monumental change occurring nearby generally you can go one of two ways, the 5 W's and one H. Or The senses.
Who
What
When
Where
Why
How
That format is generally used when characters are meeting for the first time. It is not uncommon at a first meeting for someone to review all of their time settings or to reacquaint themselves to their environment with a person as well. Let's break it down.
Who, Who is your character meeting, are there rumors you've heard about them? Do you not like their first impression. Remember First impressions are usually determined within the first 30 seconds of a meeting, so it's important to react accordingly.
What, What is the purpose of this meeting? Are you two tributes who know little about each other? What will be your course of action to make your first impression?
When, What time is it? Are you late for something? Early? Did you happen to meet this person by chance in the street because you went to school earlier or later than you usually do. Time is always a big factor in novels. Make sure to throw it into your post every now and then.
Where, Where are you? Where is that snack machine you came here for? Where is your boy or girlfriend? Placement and setting play a HUGE role in every post you make, don't neglect it or it will make for a dry roleplay.
Why, Why are you still here in this situation? Why did you bother coming here in the first place? Why bother. Why is seen as the biggest question on the list. Make sure to at least reference it or your post structure will have a hard time holding up.
How, How did you get here? How is it you are still alive? How do you plan on putting a knife in your buddy's back? This is a broad question and need not be answered every time, but it's always nice to throw it in every now and then.
The other post format I use is what I call the 5 senses. It's where you go through each of the senses you possess and then right them out as your character. The trick here though is not going through your sentences and just talking about what you hear and see, it's the transitions and making your post flow smoothly. Which is something that you can define by your personal writing style alone.
Hope I helped, if not, Bacon. And now that I've definitely helped, have a good day.