I am going to contradict one of those "rules," the one that says that you can't put a preposition at the end of a sentence.
Now, for those of you not sure what a preposition is, it's a word that describes the relationship, often in time or space, between things or ideas: "the book is on the table". "On" is the preposition, as are "to," "for," "of," "by," "around," "beside," "with," etc.
The idea that one cannot put a preposition at the end of a sentence comes from Latin, a language whose rules governing syntax (word order) are VERY different from those of English. However, to apply the Latin rule to English does a disservice to our own language.
Certainly, the formality of writing is elevated when one "writes around" the preposition, which may be a desirable outcome. For instance, "he's the person I told you about" becomes "he's the person about whom I told you."
However, it can also create awkward or unbalanced sentences, which are decidedly lacking in elegance, a point aptly illustrated in a quote attributed (probably apocryphally) to Sir Winston Churchill, asserting that "Ending a sentence with a preposition is something up with which I will not put!"
P.S. (February, 2012): The fantastic web-zine, Slate.com, has created a language-related podcast they dub, Lexicon Valley. Lo and behold, this one addresses the origin of this grammar "rule" forbidding prepositions at the end of sentences. Listen and learn how this myth took shape.