I have two campaigns on two different sites and I need players. So how do I promote my games in order to get more players?
The most important thing you must do is keep the games active - even before anyone else starts! It might mean that you have to have 2 or 3 NPCs that post regularly. Without some activity, no amount of promotion will get players to join in on a dead board. It might seem odd for them to join in part-way through an adventure but PBP can get away with that. Famous RPG GMs/players with some clout
Most big RPG forums have a promotions forum, I'd advertise in there. I would also create a nifty graphic that represents your campaign. Gamers are enticed by imagery and if you find a strong one then they will flock.
Finally, I would keep the proposition simple, fantastical and easy to understand. If you mention lots of names of places and people then it is difficult to understand. For example...
Drankinott, dark evil of Blagmarrion must be stopped from raiding Brantinor of Moriblock, else the Skorgbart of Changitott will reign his Blobbomerry...
...is difficult to understand but instead...
Stop the evil Lord from raiding your quaint village that nestles in a lush green valley before your God comes down to serve you your posterior...
Here's how I promoted my campaign in a FLGS--that's Friendly Local Gaming Store for the acronym-challenged.
The right venues can be digital as well and the right message is critical. While the below flyer was put in brick and mortar stores, with a little imagination one could craft the same for digital venues that sell players your campaigns. A web page or forum sticky post comes immediately to mind.
The question is "how do I promote my games" not "where do I promote my games." In answering, it is necessary that the message and presentation, digital or paper, be attractively presented. My flyer works in digital form too: instead of tear-offs, provide a dedicated email box to collect interest threads and follow up replies quickly and answer questions both from email or on forums where your campaigns are promoted.
Players want to be excited about the game and get a taste of the GMs dedication and responsiveness. Play-by-Post often requires attentive GMs and players and timely replies in the forums and to email will be critical in the soft impressions that players use to build interest in your campaign.
For example, my tear offs take interested parties to a dedicated wiki. Your digital promotion could do the same, but to the forum where the Play-by-Post will be hosted (or a sticky post with additional information and FAQs).
Regarding the presented flyer: feel free to steal and borrow for your digital marketing effort.
The most significant way to recruit players is word of mouth. Ask your friends (and twitter and facebook are very good for this) and once they start playing tell them you are recruiting. Most gamers know other gamers and can help you recruit, and there is no better way to persuade someone to give it a try than having one of their trusted friends endorse it.
Beyond that, I will second what @Rob Lang said and point out that many forums have a promotional section where you can legitimately talk about your game and recruit and agree that having an active list/forum helps even before you have a single player. You can post background material, pictures, actions of NPCs, etc even before the campaign starts.
And, as an aside, I will say that I personally prefer PBEM over play by post. It just works better with my software setups and preferences, and with a decent list manager like Yahoo's you can have the best of both words since the mailing list manager will keep archives for you and make them accessible.
I was able to get a couple players through PenAndPaperGames.com. But i would say word of mouth is better - if someone knows you and your game style and thinks they will be a good fit, theres not much better vetting that you can get other than direct knowledge of them. I post my campaign notes online - outlining the setting, what its like, and some maps and histories - the more interesting you can make it, the more likely you will attract good players. I dont get a thrill out of anything but real table top, but if PBEM is your thing, theres a LOT of sites for that.