While PbP play has some challenges and disadvantages in comparison to live table gaming, one of the places it shines is in permitting the use of audio and visual gaming aids. These can be used around a live table, of course (particularly if you have some sort of fancy wired gaming table), but they integrate even more seamlessly into the PbP experience. Don't let this advantage pass you by.

Pre-Gaming

Audio/visual accompaniment can help set the tone or provide inspiration for your game before the first die hits the table...err, before the first person clicks on their virtual die roller. In fact, using this method can help overcome one of the tougher problems in PbP game (and RPGs as a whole): getting players on the same page about tone. All it takes is to search around for a few pieces of media of whatever type that matches the tone and feel that you want for your game, and point your prospective players to them. This also works for players: use the same technique when trying to communicate about your tone expectations during the pre-game discussion.

Want to run a gritty modern crime game? Go to Youtube, find a couple of clips from Heat or The Way of the Gun or Collateral and put the links in your Recruitment thread.

Looking for an epic-feeling fantasy game? Dig up your favorite epic fantasy soundtrack (The Lord of the Rings, 300, whatever).

Player expectations for the tone of a game are frequently set by media. That can be an advantage in setting group expectations, so long as all of the players are looking to the same pieces of media.

A Picture is Worth a Thousand Words

Painting a clear and compelling verbal picture is a critical skill for PbP players (and especially GMs). However well-developed your skill in this area, though, no verbal picture is going to do as well as a picture or map in communicating the information about the look of a scene or the spacing of objects within it. Particularly given the ways in which written descriptions can be subject to varying interpretation (particularly particularly when not all of the players may have the same primary language), pictures and maps can reduce confusion and create a more exciting and evocative mental space for the game.

The same considerations go for people, other creatures, and objects: a picture can not only save a thousand words of description, it can help ensure that all of the players are seeing the article the same way.

This isn't just GM advice: every player in a PbP is going to be describing something at some point (even if its only their own character): it's worth taking the time to poke around online to see if a picture of what you're describing already exists.

There are no end of public domain pictures on the internet that can be used to liven up and bring fictional consistency to your game. If you're reading this, you're presumably internet-savvy enough to not need advice on finding them, but don't overlook the simple expedient of Google Image Search with a brief description of the thing of which you want a picture.

Maps are a little tougher to come by - though there are any number of free maps available - but don't overlook the electronic versions of RPG maps for sale online at outlets like Drivethrurpg/RPGNow. Electronic maps are available on these sites in all price ranges.

Once you have the picture or map, deploy it in game in the way that is most convenient for your players. If your forum software allows for in-post pictures, use that capability. If not, link to your image host of choice (Photobucket, Imgur, whatever) and put the image there. Also, don't forget to also post the picture or map to your game's wiki or other resource page so that you and the players can refer back to it easily, without having to hunt down particular posts.

Soundtracks and Ambiance

Nothing sets the ambiance like music. The use of music is one place in which PbP has an easier time of things than a live table game. While there are GMs who can do it well, generally speaking music at a live table game is a tough element to integrate. Beyond the issue of noise and distraction, there's also the logistical/practical concerns with getting speakers arranged and the right music playing. None of those concerns are present in a PbP game.

Be it Youtube videos, Spotify playlists, or one of the myriad other sources of publicly-available music on the internet, linking to music in your posts can be a powerful way to inform the other players of the tone or setting of a scene. Player characters entering a nightclub? Link to the music that they'll hear inside. Whimsical chase scene? Yakety Sax is right there. Your character is in a band and playing a gig? Link to the type of music they're playing. Creepy scene in a horror game? Pick a piece of music from a horror soundtrack and tell the players that it's the background music for this scene.

Some of these techniques will not be appropriate in every game, of course, but the overall point is that music can be your ally in a PbP game in a way that is tough to realize around a live table. PbP has enough disadvantages compared to live table games: don't overlook the places where it can do things better.

The mechanics of integrating these items will depend on the software of your PbP forum and maybe the law where you live, of course. But, within those constraints, non-verbal media can be a big enhancement to the PbP experience.