1.) compounding two (or more) words together: houseplant, redheaded
2.) using prefixes and suffixes: pseudoantidisestablismentarianism
3.) borrowing from other languages: vampire, ghoul, paradise, robot.
4.) Attributes of famous people: mercurial, Jovian, Saturn, Newtonian, Christian.
5.) malanthrope, using an existing word in the wrong way until its accepted: gee, that was neat.
6.) Onomapapia, using the sound of something to stand for it: gallop, stomp, whistle, growl.
7.) Using a phrase from an older more respected language (latin or Greek) solar plexis, appendix, cardio. digitalus.
8.) Turning a noun into a verb: blogging, networking, cybernization.
9.) Or just making up a word to serve a need: robotics, Polonium, Grot.