A good, simple format for an OoC opening post (an interest check should follow a similar structure - it's just I personally tend to patrol the actual RP section for things I might like, and only very rarely remember the interest check exists) should go something like that: 

[quote][b]TITLE[/b]
[i]Catchphrase or short paragraph filling the same role.[/i]

Premise (around 1-3 paragraphs about the plot and what the player would/could play as - what to expect; *not* in hider. Keep it brief - you want people to be able to quickly skim-read it so that they can decide whether or not the RP might interest them and they want to delve deeper).

Link to other, more elaborate info. (The character limit on this site? It is direly insufficient for anything even slightly detailed...)
If the world is very detailed, preferably make it two sections - firstly a fairly concentrated listing of things anyone who joins should know of the world, and then the in-depth workings that can be referred to at necessity (or read at one's own leisure). 

Summary of (major) happenings this far (constantly updated as the story goes on, preferably in hider to avoid extra scrolling).

How to apply (character sheet templates and guidelines etc.) + rules. - Rules perhaps visible, templates preferably hidden.

List of characters currently in RP (Named link to CS + short identifier sentence to each; visible.)[/quote]

Graphics can be and often are a nice addition,[b] but they should be kept simple and not overwhelming.[/b] Make the title a nice complementary banner, give the sub-section headers a bit of extra size and color so they can be spotted with ease, such things. 
[b]Do not[/b] use huge, page-stretching pictures, bizarre text structuring (nice paragraphs, please - no gapped list of single sentences, and no uniform walls) or alignment (is centering text really necessary if it is not a poem?), unreadable fonts or neon/backgroundlike colors (people's poor eyes!).