The hot glue strings method it's very good but it's time consuming to achieve a realistic loock. There is a device to attach to a regullar electric drill that makes really impresive spiderwebs using a special fluid sold with the device. I't a cobweb spinner.
http://www.yardhaunter.com/halloween_cobwebs.html
The source page of the images explain another method using compresed air and hot glue:
quote:
COBWEB SHOOTERS
Ever wonder how haunted attraction spin their cob webs? Opposed to the unit above, these are called Cob-web Shooters, and that's exactly what they do.
Cob-web shooters spray out realistic looking webs much faster than fan type web-spinners. The base unit is a highly customized hot glue gun, equipped with a special tip that transfers the webbing material into a stream of compressed air, allowing for detailed application of cob-webs. These types of web shooters do require an air compressor with up to 100psi.
Whether you need close-in webs for detailed work, or are shooting webs over the front of your garage, this unit can do the job. Using the flow-valve, you can adjust amount and density of the cob-webs produced. The web material cleans off easily, but we always recommend that you test some on important materials. This unit uses regular hot glue sticks to produce lots of great looking cob-webs. Or, you can buy special glow-in-the-dark glue sticks that glow under Black Light. Another nice benefit to these types of web shooters, is that you don't have to worry about the web fluid drying up over time.
Minions Web actually has detailed instructions for building your own, or if you prefer, you can buy a ready-to-go unit for less than fifty-dollars from them.