I’ve been asked to sketch a tattoo for one of my friends. I’ve got a general idea of what I want to draw, but I didn’t want to go any further in my sketch if it’s not going to work at the tattoo parlor.
I’ve heard random things like, "don’t put too much detail into the artwork!" or "make sure there’s lots of space for the tattoo artist to color in"
Anyone have other pieces of advice? Thanks! 
Tattoo ink tends to expand slightly once it’s in the skin – if you have too many details or too many small spaces, the design may become blurry in those areas once it is put on in tattoo form.
If you have any experience with pointilism, that seems to be slightly akin to the experience you get when you tattoo, so you could try designing in that technique to get an idea of what will work and what won’t.
Other than that, I don’t have much input for you. As an amateur artist, myself, I have designed a half dozen or so tattoos for friends and family and have followed these guidelines, and in my opinion, all of them have turned out well.
Maybe you could take your tentative design to the artist doing the tattoo first, and ask if they have any changes to recommend before actually putting it on….
Hope this helps. Good luck!