Great replies here., maybe this should be become part of a FAQ
There are some things to remember, and consider. Your market may be very different than others that post here, in fact it will be.
We as a group would need very detailed info to respond better. If you are in Atlanta that is different than if you are in Woodstock.
Is your area growing, ie, new building being put up? If so, the demos are very different than if it is a built out area and has older apartments. I sort of agree with Mike on the census issues, and the others on how to find, except to look in the white/yellow pages.
For census http://www.census.gov/main/www/cen2000.html
This will take you some time to figure how to get around, hope you have cable or DSL. Find your census tracks and then you can see, providing the tracks are uniform, what is around your mat for people. I have 1 mat that has all little pockets of census tracks, I was trying to use the census on that mat to determine what another might do in a different area. It does not always work. Of course you can BUY data that is a cheap investment. What to do with it is another question. I happen to know what the average mat in my area does for volume, and the density, so I have an idea of what to expect per population in a given area. But this is over years of time I have this info. And it won't work for you, you need to know your local market. I do the "drive" method the most. Look for small apartments, duplexes, trailer parks. These people probably need a l'mat, and likely you will find one nearby... If you are looking much more than 20 mins from your home you are looking to far. You can always ask existing owner if they are thinking of selling. If you are in a bigger city, business brokers, the paper are ways of finding mats. New mats have a place, but must be picked very carefully.
Also, money is key to success, you should figure investing 150% of your own cash, what you want to make yearly, though a home equity sort of works, but still can come back to haunt you, then in 5 years you will have that. If you want to make 20,000 a year then you really need 30,000 cash, and you won't see the 20k for 5 years. I hear laughing in the background from those in high density areas...