Brother, it all depends on what you want.
How big of venues do you want to fog up? What do you want the fog to look like- do you want it to be an ominous cloud, a mist near the floor, or just a general ambiance? They make different foggers for different prices that do different things.
For what it's worth, I've had great results with Halloween foggers. They're cheap (especially on Nov 1) and they generally do a good job. If you can, grab 2- one for each side of the stage.
OK, now for the fun part. If you want the billowing cloud of fog, let it run stock. Be aware that sometimes the fog will actually block people's view of you. If you want general ambiance, just put an inexpensive fan in front of the fog spout. It will disperse the fog more quickly and you won't have so much of a cloud.
If you want the fog to stay low, well, there's the dry ice method... but if you want to use the same fog machine, you might try building a
fog chiller- that's all the "pro" level low lying fog machines do, anyway.
Now, if you're looking to fog a big location, or if you want a lot of fog pumped constantly, the Halloween guys won't cut it. At that point, you'll want to look into something like an American DJ Fog Hog.