As comprehensive as the existing set of symbols is, it doesn't have every possible symbol that could exist, and it really takes the fun out of inventing a new symbol if you can't typeset it nicely.
One option I know of is "combining" existing symbols in some fashion, such as negative spaces. So I could write something like
m \gg\!\!= k, but that technique has obvious limitations.
What's the best approach here? ("Stop making up new symbols for no good reason" is a valid answer.)
Edit: For a bit of context my actual interest is in nicer display for typeset source code, particularly making common operators written as multiple ASCII characters look more like a single symbol. This often results in symbols not otherwise used--but recognizable to other programmers--and for which the negative spacing technique suffices. As mentioned in a comment on vanden's answer, I was curious about the more general case.