When posting, is it generally normal to submit to more than one category on your blog if the categories are relevant, or would you only post to one category on your blog?
I post to one category but use your tags to make sure you have it also linked to other keywords/categories. Just an idea.
I try not to submit to too many categories (max. 2) because otherwise categories just become irrelevant... But using tags is a great way to be more precise.
Use your tags rather than categories. This have two plus points. Most themes use categories as navigation and with too many categories, you'll have a very hard time maintaining the navigation properly. Second, if you have set archive as do follow, then having too many categories for the same post will hamper your rankings for double content and stuff however, tags are usually cluttered and not used for navigation. Plus, you can set tags as do follow or no follow and this wont hamper your rankings as for a single tag the possibility of different contents popping up is quite natural compared to that of categories