A day or two before you want to fill the nail holes, put a small amount of your wall paint (about 1/4 cup) into a small seal-able plastic container. Leave the lid cracked a bit so a small amount of air can get in. In 24-48 hours the paint turns into a thick paste, almost the consistency of spackle.
Then just use a putty knife (or your finger) to fill the holes. As soon as it dries you're ready to go! I like this method because it eliminates having to putty and let it dry completely before painting. I need instant gratification!
Another very simple solution to filling nail holes: toothpaste! As long as it's the paste type and not the gel, you can use it to fill in holes just like putty. White toothpaste is best, but in a pinch you can use green. Fill the holes, let it dry, then paint!
I hope these simple tips help, and happy decorating!