What are some simple tips for making my book look good?

You want to appear professional, and there are some standards that apply to the reading experience that may be little things in and of themselves, but readers will notice them (even if they can't tell what it is they are noticing) and will be able to tell if a book wasn't professionally produced.  So, here's some simple tips that are really easy to implement in your book to make it more professional that your competitors may not even think of (which means you'll have a higher quality product!)

• Always justify the prose in your books. Obviously, chapter headings, title pages, items in the header and footer of your book may have different styles, but for the main reading experience you want to have your pages justified.  Unfortunately, Microsoft Word doesn't do a very good job at this; it does a passable job, but it won't automatically hyphenate words to prevent large spaces from entering your lines between words so depending on how thorough you want to be in the final typesetting phase, it may be worth looking for those unnatural spaces. Leaving them in isn't the end of the world, however, and they're more likely to be ignored or missed than if you left-justify your book.
• Remember that odd pages are always on the right! Page 1 should be the first page of your book (which should not include the front-matter!) and that it should always be on the right side and followed by pages 2 and 3, then 4 and 5, etc. Even on the left, odd on the right. Microsoft Word doesn't have an option for viewing your book this way, but after exporting a PDF you can usually view it like that to make sure everything looks right.
• Don't forget to include the front matter before the start of your book…at a minimum put in a Title page (which should be on the right) and a copyright page (which should be on the left, usually the page right after the title page.)  Optionally, you can also include testimonials (which I normally put before the title page), a table of contents (after the copyright page) and/or a dedication (which I normally put right before the actual start of the book.) Use a section break and reset your page numbers after the last page of your front matter so that your book starts on page 1.
• Watch your blank pages: There should never be a blank page on an odd (right hand) page, and they should be blank – there should not be a page number or header or footer on the page. You can accomplish this using the “Page Break – Odd Page” option in Word to skip to the next right hand page automatically after the end of each chapter and after your front matter.
• Make good use of white space and include ample margins, especially in the gutter of the book, which is the space in the middle where the book is bound (the right hand side of even pages, and the left hand side of odd pages.)  At a minimum make sure that there's enough room to keep print from becoming hidden in the binding or by somebody's fingers as they hold the edges of the book.
• Be consistent…and this is a big difficulty with Microsoft Word. Apply styles to everything and resist the urge to change the font for a section manually – that way you can update the entire book at once by updating the style's font. Try to limit yourself to 2 fonts in the interior of the book without a good reason, one for the chapter headings and one for the body of the book. Make sure that any headings or page numbering that you apply appear the same on every page, not including front or back matter or blank pages.
• Use proper case – sentence case for the body of your work (an initial cap followed by all lowercase letters except for proper nouns and acroynms) and title case for all titles, subtitles, chapter headings and subheads (an initial capital for each word except for short prepositions.)

That may seem like a lot, but after a book or two it will be second nature and it can help the appearance of your book a lot.  If you follow the above rules, most people may not even realize your book isn't self-published even if they are looking for that sort of thing.