Also, this post refers to the ABSOLUTELY FINAL DRAFT. The one you are so sure you got right that you are prepared to click "submit file" and put it up for Amazon, no more questions asked. You have gotten it peer reviewed, had others critique it (maybe through an online site such as Scribophile). You have prepared and formatted and are ready to go.
Here are some things that are easy to miss, things that might not even be there until AFTER you hit submit on the Createspace site (or whatever site you are using. I only have experience with Createspace, so other sites might be different.).
I already gave some tips on this subject in my "How to Use Createspace" post (here), but at that point there were things that I didn't even notice.
1. When you upload, even if you used their provided template, Createspace does a conversion that can slightly alter things like margins which in turn can change things like page layout and page numbers. Even if you have double checked and triple check your personal version for things like unwanted spaces between paragraphs or empty pages, double check again. Just because they weren't in the file you uploaded doesn't mean they haven't suddenly appeared.
2. Because of what I mentioned in item 1, page numbers can be off, so if you included a "table of contents," it is wise to go back over the piece and check that every chapter does indeed start on the page number you listed. This one was something I didn't even THINK to look at until just this morning. I was prepared to push that submit button and send my novel out there. I had NO IDEA every single page number on my table of contents (except for chapter one starting on page one which is a given) was off.
3. How easy to read is it? For this I definitely suggest splurging to get a hard copy proof (as seen here.), unless you are some sort of super genius who can tell exactly what a printed page will look like based on the computer screen.
4. Scan it. Look for big blocks of text. This is generally a sign that you need to break paragraphs up into smaller sections.
5. Are the spacing and font choices consistent for Chapter Headings? You don't usually see these side by side so it is an easy place to be accidentally inconsistent.
Those are just some quick tips of things I wish someone had warned me about earlier. As promised, there are some links below for "general proofreading advice." Good luck and have fun out there!
PS I found that some of the page number problems can be avoided simply by choosing to upload as a .pdf rather than a .doc, so if you are having formatting difficulties try switching your file type and see if that fixes things.
Grammar Girl/Quick and Dirty Tips
Writing Center-UNC: Editing vs Proofreading
Daily Writing Tips: 8 Proofreading Tips