Well I have googled upon for a while and come to know that a few techniques are considered unethical. You have to be careful when you use then. I would list these techniques as:
- automatically generated doorway pages - false redirects and cloaking - keyword stuffing - hidden text - pages loaded with words making no sense - duplicated content - misspellings of well-known web sites - unrelated and centralized links - some methods that try to cheat search engine spiders - hidden links
