1. WRITE GOOD VOCAL PARTS - This one seems obvious, but so many people come to the studio the day of vocal recording and don't really know what it is they want to sing. They have ideas, rough melodies, but no real structure or pattern. One of the worst things a vocalist can do is be all over the place in a song, so much so that nobody (including the singer!) can remember what was sung. The vocal is very important, especially in pop, country, and rock music. If it's difficult to sing, that's fine. If it's difficult for your fans to hum, you're probably trying to hard.
2. PRACTICE - Before coming to record, make sure you have practiced your [well written] parts many times. One thing that really helps is to actually gig with the songs before you record them. If you haven't gigged with the song yet, you may not really know what it's like to perform it all the way though when the pressure is on. This is what you will have to do in the studio essentially, so if you come without having that experience, it may feel awkward and you may tense up.
3. MAINTAIN YOUR VOICE - This is mainly about taking care of your physical voice! People that are often sick should be extremely careful to come into the studio NOT SICK! Changes in vocal timbre due to illness are very obvious. Get plenty of rest before you come into the studio, and don't warm up too long. Usually 15-30 minutes is a perfect warm up time for most singers. If you come into the studio and your throat feels like it's coated, you can try drinking some water, and if that doesn't work, try a shot of Crown whiskey. Trust me, this isn't so you can get plastered and magically nail the take; whiskey has such a high alcohol content that it can really clear out your throat and nasal passages, making it much easier to sing. I've had some singers sound terrible and then drink a shot or sometimes just a half shot of Whiskey and their voices are cleared out and their throat feels more open.
4. AVOID "BAD FOODS" - Singing on a full stomach is hard, especially on stage. It's best to eat light before gigging or recording, and avoid all dairy products if possible. Some of the best things to eat on days where you sing include apples, meats of all kinds, fish, bread, veggies, and lots of water. Avoid citrus fruits and bananas, as well as sugary foods. Stay light on the beer as it will make you bloat and feel uncomfortable, and probably sing worse in the end.
5. RELAX AND ENJOY YOURSELF - If you're not enjoying yourself, it will probably translate and be very obvious to the crowd. If you don't like the song, figure out what you need to change so you can love it, or just ditch the song altogether. Being tense and nervous before a gig or before recording can kill the mood. The best takes and performances happen when singers release all their energy and really sell the message of the song. Emotion is priority #1!
A few videos of amazing singers to inspire you: