Some publishers have so many hidden clauses in their standard contract I'd suggest a lawyer read it so you fully understand what you're signing. Some of these I've seen include:
Saying you get a given royality (say 10% each of cover price), but in reality if a book seller/buyer is overseas, its cut in half. If the bookseller buys more than a certain number of books from the publisher at one time, its cut in half (or in half a 2nd time). So in many cases you actually get 2.5%. Same with foreign language editions, in some cases all you actually get is being able to say your book is in another language.
Have a specific date in the contract saying EXACTLY when you will be paid in each royalty period (once a year and specific date, twice yearly and the specific dates, etc).
Some publishers say (or list in their books with the publisher data) that they have an editor when in fact he just designs or does some other aspect of production. He never actually ever reads the book. Have several people you trust read your manuscript for any small mistakes, flow input, etc. After you read your own material so many times, a writer doesn't actually "see" errors or problems. These are different than hard facts. If a publisher does use an editor, have a clause giving you sole approval over anything he/she changes/edits. Editors have no clue as to what you've written about. If they did, they would have written the book. Some small publishers also think they are experts on everything and change facts to reflect "their knowledge" of a topic. As insane as that sounds, I've seen it done. You've corrected previously incorrect books and they change your material to the prior published mistages
Some contracts require first option on the author's next book, and then again with that one, and so on. So in actuality you can never break with the publisher even if given a horrible deal or experience. A clause to have removed in ANY case. You can always resign at YOUR option.
If you want any design input or say regards photo size, cover illustration, even specific title, etc, have it in the contract. A verbal promise is, after all, just talk. These can be general design or ability to prevent, as some do, photos being made overly large (and thus distorted) to increase page size or retail book cost.
Determine a price for you to buy copies of your own book, normal is 50% off. If your book costs $50 and you sell 40 signed copies directly to buyers vis forums, etc, you make $1000 profit. Looking at what some publishers actually pay in royalty, it is a considerable amount.
In all cases, demand blue line pages inspection and ability to make corrections to them before printing. If not, any mistakes you have in the book (wrong captions with photos, missing paragraphs, etc) are your own fault. Any mistakes are always pointed towards the author, even if a designer or publisher mistake. Probably the most important clause as far as the author being happy with the actual product.
One personal thing I have. Demand whatever length of acknowledgement page(s) is needed to thank those who have helped. A sentence or line is FAR more appreciated by anyone rather than just a column list of names. I consider the latter simply rude.
If you want copies sent to a specific public library or archive, have it in the contract. Otherwise they'll only get a copy if you buy one and send it to them. Likewise, very few publishers spend the money for review copies. While internet websites are a plus for free comments/reviews by readers, in the case of magazines in most cases a review copy is normally received from the author, and even that doesn't insure a review.
In the case of 3rd Reich topics, consider the cover with care. A large (or in some cases any) political emblem of the period can deter some outlets in foreign countries (bookstores or chains) from selling or displaying the book. Its the law in some countries.
Have the contract state how many initial free copies go to the author. Normal is 5-10. With every book I've done I've had to buy more than that simply to send a copy to those who helped.
Look at past actual products of the publisher. They will generally reflect what you can expect. If you expect something radically different than their norm, all points that differ must be in writing. Again, a verbal promise is simply talk.
Contact an author who has already done a book with the publisher you are considering and get his input directly regards the experience. Consider that input when reading your contract and asking for changes or additions.