1. Have the character drawn nude. 90% of the time this is a good idea. Even if the character is never to be drawn in adult works, unless you're planning on having them drawn in the exact same outfit every time, it's best to have the full body available so that any clothing style can be drawn, without having to guess what's underneath the clothing in the ref. The "nudy bits" don't have to be drawn, but if you ever plan on getting an adult oriented picture done, it's best to include them just incase.

2. Hands and feet are both fully visible, and not covered up or cropped off the page. You'd be amazed how many times I'll get several pictures for refs and every single one of them will end at the knees. Or the only picture I'll have will have the hands hidden. The hands and feet are great places for unique markings, so it's not comfortable having to guess. Make sure the hands and feet are fully visible in your ref!

3. Use poses that display the whole body. A ref sheet is designed to show the entire character, so if all you have is one view of the character and they're curled up into a little ball, a lot of information is going to be missing. Keep poses open and free so that nothing important is blocked by other body parts.

4. Keep those eyes open! More than once I've gotten three or four pictures as refs and not had the eyes open for a single ref. Not a huge deal, but having to wait a day or two to get into contact with the commissioner just so you can do a literal one second job can be mildly frustrating :P

5. The back doesn't necessarily have to be visible. For example, if your character is mostly a generic fox, most anyone will be able to figure out what the back is like. However, if they have any specific or special markings at all, make sure it's there! Even it's something simple, knowing the exact position and size can make things a lot easier.