Recently, I've been having an issue running my weekly or even biweekly Dungeons and Dragons game. With five players, it's been difficult scheduling everyone for a convenient time. And even then, work or life was often rearing its head leaving us short at the game table.
Then one of my most consistent players had to drop out of the group, leaving us with 4 players and myself. It was definitely time to turn on the recruiting drive.
There are a number of different things you can try. I'll list out what I did and what seemed to be the most successful for me.
One of the things I did NOT do, was post a sign up at my local game store. It's 2013 folks, and since I do most of the session scheduling via email, I wanted someone at least tech-comfortable, if not tech-savvy. Other folks my have better results recruiting from a game store post, but I was trying to cast a much wider net for players, and just a few small posts at a few stores in the area wasn't going to cut it.
Since I run my game in the Dragonlance setting, I thought going to Dragonlance fan sites and forums might yield a few potentials. Sadly, I've tried this route several times, and never reached a single player. When going to fan sites and fan forums, look at the focus of the site/forum. If most of the focus is on discussing fiction related to the setting, or game rules, it's probably not worthwhile.
What has worked. Sites that focus on recruiting, or social meetups. I got a number of great players over the years from the EN World forum Gamers Seeking Gamers thread. If you're looking, definitely set up a post there and subscribe to the thread so you get email updates when anyone posts to it.
Another site that has been very fruitful in finding new players is Pen & Paper games. That site also has a Seeking Players form thread, as well as allowing you to search for players based on their profile and geographic location.
Lastly Meetup.com has been wonderful. In my area (just outside if DC), there are a half dozen groups dedicated to RPGs. Join a few, and look over their forums, and meetup threads. And don't be afraid contacting someone who might not be looking exactly for your type of game. I was able to find some folks in the area looking for a 3.5 or Pathfinder game into my D&D Next game by just reaching out and telling them a little about the new system and my game. Likewise, I recruited a player who I thought might be living a little further than he might want to travel, but after talking, it turned out he was available and OK with the distance.
In short, if you're the DM, and you're looking for new players, you need to take a very active role is recruiting. Post online or in-store, and follow up when someone reaches out. You may get more misses than hits, but if you work at it, you can find players out there to bring to your game table.