Running out of Algebraic Symbols

I'm currently writing my thesis in computer science. My work contains a large amount of equations to do with linear algebra, data transformations and calculus. On receiving feedback on my first draft from my supervisor, he has told me that I am reusing symbols all over the place. So I am going back over things, but I'm finding that I'm running out of both English and Greek characters and I'm having to constantly search back through the text to see if the symbol I want to use has already been defined. For example:

Take two vectors X and Y with lengths m and n respectively.

All of these symbols have previously been used. So my question is, once a symbol has been defined once, can it not be reused for the whole thesis, and how then should I deal with this problem.

Thanks.