If you want something to show up on permalink post pages but not on ask, submit, and other non-post pages, wrap it in {block:PermalinkPage}{block:Posts}{block:Date} {/block:Date}{/block:Posts}{/block:PermalinkPage}.

1. Don’t remove theme credit. You can change the format of the credit (using an icon or HTML symbol instead of the full text) or even leave an invisible credit in the HTML by commenting it out, but don’t get rid of credit completely. It’s like going to a museum and painting over the artist’s signature, or scratching the name off of all the books in a bookstore. Rude, right?
2. Credit theme authors if you use their coding. Sure, I didn’t invent HTML or CSS. J.K. Rowling didn’t invent the English language either, but the writing in the Harry Potter series is undoubtedly hers. You don’t need to credit someone for using text-shadows or a particular combination of fonts, but do leave a note if you copied whole chunks of code. If you’ve heavily edited a theme, you can put “Original theme by ______”.
3. If someone doesn’t want to share a theme, don’t push the issue. I had my own personal experience with this - I created Tumblook a couple of years ago and people were pressuring me to release the theme. Instead I gave it to a couple of trusted friends who were well-known theme authors, and at one point considered not releasing it at all. People got angry. One person went so far as to recreate my theme and give it away to people, saying “stop kissing ass and share things”. Isn’t that my prerogative, to do as I wish with my own creations? Be respectful - no means no. Many theme authors don’t make any money out of theming - they do it for the pure joy of creation. Don’t ruin it for them by being inconsiderate and rude.
4. Understand that theme authors may not have time to help you. I am going to school full-time and working on my free days and weekends. Many theme authors are similarly busy, and most don’t theme for a living. If they don’t have time to answer, it’s not always because they don’t want to help you, it may be because they don’t have the energy or motivation. If you paid us, we’d make the time, but giving precious time away for free is a sacrifice not everybody can make! They don’t have time to make everybody a custom theme or help you with old themes they no longer support.
5. Theme authors are limited by the code. Tumblr, while being amazing, has many shortcomings. The follow/reblog buttons in the top corner of your blog may not show up, regardless of coding. Most times, the theme author has no control over this and is not responsible for its disappearance. Theme authors can’t make your GIFs animate properly or make Tumblr audio posts render on mobile devices exactly the same as on a computer. They’re not gods; they just have a knack for design.
6. Theme authors burn out. Artist’s block happens. We can’t chug out a theme every week without sacrificing quality. Many would prefer to wait until they can give you something 100x better. Don’t rush them. If they share previews, like I do, I know waiting can be difficult, but worth the wait!
7. Theme authors have their own way of doing things. I limited banner options on Greying to make content more accessible - if you have a large fixed banner and somebody views your blog on a small screen, scrolling is useless. I rarely hide image captions or sources because I believe it’s unnecessary. I leave previous and next buttons intact in case the inifinite scroll plugin breaks or doesn’t load properly. If something isn’t supported on the major browsers, I tend to avoid using it. I may not help you change these things, but I have a legitimate reason for doing so!

Take these things into consideration when you use a theme or are in contact with a theme author. The more happy theme authors are, the more likely you will continue to receive awesome themes from them!