For me personally, I like to have my OOC ready before my interest check. A lot of people might see this as a wasted effort, but I've seen too many RPs stall out between gathering interest and building their OOC. For me, the interest check is a way to advertise as opposed to gathering enough interest to run an idea. If I have an idea, then I'm going to run it and I think that confidence goes a long way to getting players interested. 

That said, keep the iCheck concise, it should be able to communicate your premise but it does not to include the entirety of an OOC. The OOC on the other hand should contain or at least direct to all relevant information a player will need to create a character. If it's a fandom RP, link to relevant wikis or characters where appropriate. If it's original, lay out your information logically and with enough detail to create your world. If you want players to contribute to world-building, leave them space to do so, and if not, communicate that as well.

Have rules in place. But more importantly, make sure they are rules that you will follow. If you apply rules to a game that you as a GM break, then your players' trust and faith will be quickly eroded. This is where you also get to post your desired pace and post length, make sure it's again something you can produce consistently as well.

Character sheets don't need to be overly complicated, nor do they need to be long. Make sure they include information relevant to your game and what you need as a GM to accept them. A lot of character sheets are bloated with information that is not relevant to the game or would be better revealed in the IC. I'm pretty guilty of overly convoluted character sheets myself and a lot of times too much information has come back to bite me. If you want your small group to primarily interact with each other in a linear game for example, don't add a supporting cast section to a sheet because that gives the impression of a more sandbox story. 

As a GM you also get to decide if you're going to play a character, just drive the story or both. You can drive via your character's POV but you risk becoming the main character. Likewise, you can drive the story through updates and NPCs but you risk a lack of engagement. I like a balance of both, but it is a balancing act where you are writing two distinct styles of posts. 

If you're using dice and rolls to determine player actions or outcomes, that's also going to factor into how you approach IC posts versus a more collaborative writing environment.

OOC engagement also goes a long way. Encourage players, thank them for playing and posting. Tell them what you liked, speculate and spitball with them. This can either be done with the forum OOC or you can do a Discord if that's more your speed.

Lastly off the top of my head. Don't be afraid to say no, whether that's to accepting a character or even a player you don't like, to shooting down ideas that conflict with your vision for the IC. There are times to be flexible and constructive, but your enjoyment also matters and as the GM you need to be engaged. If players are running off with themes and ideas you're not enjoying, your engagement will dip. But likewise, it's important to indulge players as well. You'll find a balance as you get along with your players.