No market is too small to consider. Granted you may not want to spend thousands of dollars on a market with only 50 people, but you might want to spend $50 or even less.
Make your marketing fit your book, but never make your book try to fit the marketing.
Write the book you want to write and ignore trends or market expectations. You can always find a market (no matter how small) for your book after you’ve written it.
Keep your cover letter as focused as possible. Concentrate on your manuscript and not on other (less relevant) things.
Do not take rejections personally. Many market factors go into a rejection, so it is never a rejection of you as a person or even necessarily your writing.
Try not to be too jealous of other authors’ success. After all, it’ll be your turn one day.
Use your blog/website/twitter/Facebook Page to build a platform relevant to your book. After all that time you’ve spent on it, you are probably an expert on some issue. (Even if it’s something like whether or not female armor should have boobs.)
Always do a little online legwork before attending a conference or convention. That way you won’t miss anything while you are there. Familiarize yourself with the location of the bag check at a conference or convention like ALA or BEA. You’re going to need a place during the day to store all the free stuff you get.