Beginning a sentence with a mathematical symbol This is more of a mathematician question than a mathematics question, but I hope it is still appropriate.
Several times now, when friends have been editing my mathematical writing, they have pointed out instances where I began a sentence with a mathematical symbol, such as $X$, or $a\in A$, etc.  This is the sort of thing that never struck me as inherently bad writing, since it is often exactly how I would speak the sentence aloud.  However, I can also see their point, in that a sentence beginning with a mathematical symbol can look a little weird (especially when it starts with a lowercase letter), and doesn't quite read the same as a sentence beginning with an English word.
However, now that I have been trying to avoid doing this, more and more I find myself writing awkward sentences to avoid the most natural way of speaking, and I can't quite decide if I am increasing or decreasing the quality of my writing.
Do people have strong opinions on this?  Do people have tricks for dealing with this, besides a succession of empty and synonymous clauses ("Thus", "Then", "Therefore", "We find", "Looking at"...)?
 A: This seems widely considered as bad form within mathematics papers.  In some cases, it might not be clear that the previous sentence has ended, particularly if it also ends in mathematical typesetting, or the punctuation itself might be considered as part of the mathematics.  This applies more generally, in that one should avoid placing too much mathematics around all punctuation marks.
There are also capitilisation considerations, e.g. starting sentences with f(x).
As already noted, it's usually very easy (and often desirable) to avoid this situation.
A: I personally find that it is occasionally convenient and natural to start sentences with a mathematical symbol, but I have had coauthors who do not like it, and in this case I typically modify the sentence (into what I sometimes feel is more awkward).
Tara has mentioned a nice trick for amending sentences starting with a symbol and this is typically the approach I have taken.  Another approach that I find sometimes works is to consider merging the sentences (perhaps with a little restructuring) so that the symbol follows a comma or a conjunction such as `and'.
I probably have an overly relaxed attitude towards the written language, but in my opinion, if it works and it is clear, do it.  I can envisage situations where starting a sentence with a symbol makes the text confusing to read. Obviously it should be avoided in this case.
A: Since you almost never have a sentence like "x is in A" without an additional clause, it shouldn't be hard to reverse the order of the sentence.  For example, "u and v are harmonic since f is analytic" could be changed to "As f is analytic, u and v are necessarily harmonic" or a number of variants thereof (since, as a consequence of, by virtue of).  
If there is some case where you would actually want a standalone like "x is in A" --- for example after a long lemma establishing that x is in A --- one could use flashier phrases, e.g. to wit, that is to say, whence etc.
Good writing is a lot like good mathematics: it's more art than science.
A: I also try to avoid starting sentences with a mathematical symbol.  I feel strongly about it, but I can't quite articulate why.
One trick I use that you haven't mentioned yet is something like, "The space $X$ ..." or "The function $f$...".
A: Under no circumstances can you have symbols then punctuation then other symbols.  As a special case you can never have symbols at the end of one sentence and the beginning of the next.  Outside of the above case, starting sentences with symbols is bad, but occasionally all other options are worse.
A: You shouldn't do it.  This is covered in Halmos's justifiably famous article "How to write mathematics". (You'll find lots of copies of this on the web.)  It's very closely related to the rule that you should avoid having notation separated by punctuation.
A: If one stoops to starting sentences with a symbol, then one
soon descends to finishing a sentence with a symbol and starting
the next with a symbol. Then one is liable to finish a sentence
with a symbol and start a sentence with the same symbol.
To the reader (if you still have one) madness beckons.
A: I share the dislike that many people have expressed for starting sentences with symbols. In trying to explain why, I wanted to come up with an example that would illustrate the temptation to do so, but I find it surprisingly hard. Suppose, for instance, that f is so clear from the context that to use Tara's trick and start a sentence with "The function f" would be ridiculous. (In general, by the way, using Tara's trick can add clarity by reminding the reader what is what, so I am talking about a special circumstance here.) Now suppose that we want to say something about f, such as that it is a homomorphism. It is very hard to think of a natural context in which one would actually want to start a sentence with "f is a homomorphism". Nearly always there would be a reason for saying that, or a justification for it, such as, "It is not hard to check that", or "We have already seen that". And if it came out of the blue somewhere, then one would want to signal a slight change of subject with a word such as "Now" (a trick that has already been mentioned). So I'm tempted to say that if your prose is flowing properly the problem shouldn't arise, or should arise very infrequently.
