Other answers have addressed things to focus on in the music. That's important too, but another big waster of studio time (with the meter running) for me has been studio conditions.
Playing live in your living room, listening to everybody else and adjusting on the fly, is very different from listening to a non-reactive track through headphones. Singing into a mic (that has to be kept at a precise distance) is different from singing into the room and being free to move around. Not being able to directly hear your own output (because of the aforementioned headphones) makes it much harder for you to know how you sound.
So try to practice under simulated studio conditions. Record the lead track and then, individually, practice against that, through headphones. Record that and see what it sounds like. Note what is distracting or frustrating you and investigate ways to fix it in your living room so you'll be ready for the studio.
If you're going to be singing into a mic on a stand (as opposed to one you're wearing), practice with a mic at home. If you don't have a mic, simulate one, mark the distance you need to maintain, and practice singing, with somebody watching who can point out every time you stray out of mic range. Make sure that you will be able to stand still enough to do this without compromising quality. (Again, record it so you can listen and evaluate.)
Finally, practice jumping into your music at random points for patching otherwise-good tracks. Can you sing that from verse four, line two, right now? With the right dynamics, texture, etc? Can you redo that guitar lick without playing the previous minute's worth of lead-up? Can you play at exactly this tempo to redo your intro?