After eight hours urine will start growing bacteria. You need to freeze the urine if you're getting it the night before.

Best way to thaw frozen urine, is to place it in pan of hot water. You can use hot water from the tap, or boil some water on the stove or in the microwave.

Submerg the container with the urine into the hot water till it thaws and reaches the desired body temp (between 92 to 98, I believe). Use a digital thermometer (just the tip, don't touch the bottom of the container holding the urine or you won't get an accurate temp)

I've heard that handwarmers can get urine too hot. If you're using handwarmers, make sure they are away from the body so you don't burn your privates.

I've read of some people just crotching the container and wearing a couple pairs of whitey tighties to maintain the temp of the container, with or without the handwarmers.

A lot of places aren't selling hand warmers yet cause its summer.

You need to practice, practice, practice to get the temp right and pouring it into the specimen cup. I've heard of some people who were able to maintain the temp just crotching it (or in a sports bra under the breasts if you're a chick; chicks can also crotch in the vaginal cavity, also), even if they have to sit in the waiting room for an hour or more.

You may want to practice both methods - handwarmers if you can find them, or just crotching after the sample has reached the right temp using the method above without the handwarmers, but using your body heat.

If you're a guy, placing the container under the testicles is a good place to hide it. I've heard of people who use condoms to carry in a sub, but that sounds rather complicated.