Don't Get Banned by Google
There are many Black Hat techniques that folks use to influence Googles ranking of these sites, the easy thing to keep in mind is they USUALLY DO NOT help your ratings, are simply just a waste of your energy and ultimately will result in a Google ban for the site. Sure you can find sites that use these techniques and so are getting away with it however they will eventually get penalised as Google always manages to get them eventually.
Avoid using these techniques :
Alt Image Tag Stuffing
This is really a way that webmasters make an effort to 'force fill' keywords to their website, thus the various search engines can easily see these keywords and their website visitors see no difference at all to layout and an excessive usage of certain words.
What happens is that webmasters insert graphic's that relate with their subject and add an alt image tag to it. The various search engines see this but a visitor is only going to see it should they hover on the graphic and a pop-up can look.
What you have a tendency to find is that mis-informed webmasters add multiple cases of the keywords they are targeting with a view to increasing key term density once the search bots go to the page.
The real reason for an alt image tag is if the website graphic doesn't load then your text can look rather than the graphic, that is particularly important in case a visually impaired visitor reaches your website.
My advice is always to keep it simple, use your keyword once and utilize the tags to provide a precise description of the graphic.
Hidden Text is merely text that people to your website cannot see if they visit your webpage(s). Its useful for exactly the same reasons as described in the alt tag example. All that's involved may be the webmaster adds the written text (usually keyword spam) to the page in exactly the same colour because the background and therefore it becomes invisible to the eye but gets found by the internet search engine bots.
Meta Tag Stuffing
Meta Tags are vital elements of the constitute of a website but again abusing them with a view to influencing the internet search engine bots could cause you problems. Simply the keyword tag is packed with hundreds, even thousands, of the same keyword in to the tag. For instance, the tagetted keyword is DVD and the metatag appears like:
The important things to keep in mind is that Google will not utilize this tag to rank your page because of wide scale abuse why bother, additionally it is a fairly ridiculous move to make.
Title Tag Stuffing
The same principle as above but utilizing the title tag, use your keyword once in the title and ensure it is a precise description of the page and its own content, that's sufficient.
If you need to resort to black hat techniques then your set up of one's site is wrong and they'll eventually result in the website being panalised. Its a pure waste of effort, they dont work and may kill your website.