How to switch between iMessage and SMS?

Having recently updated my iphone 4 to iOS 6, I'm relatively new to iMessage.
On my university's campus, I'll be in places where I can't receive signal so I have to resort to iMessage. I will turn on my Wi-Fi in hopes that I can use iMessage but my texts will try to send by regular SMS, but I can't since I have no signal. I keep my iMessage on and the option to send SMS when iMessage is unavailable on, but it won't switch back.

How do I switch back and forth from SMS and iMessage? When I have no signal, I want to be able to turn on my Wi-Fi, and have iMessage sending my texts for me. Then when I do get signal, I want to turn off my Wi-Fi and have my texts sent by SMS.

After 10 minutes of trying to send a text message, I put my phone away. 10 minutes later, I checked my phone and it was using iMessage. I don't understand.

IMessage doesn't use your minutes whenever you need to send a message to someone else with an iPhone or iPod Touch. SMS's have to use your minutes to send texts to other people. When you're in a place with no cell reception, you should connect to Wi-Fi and send you'll still be able to text people thanke to iMessage. It just makes texting people easier, that's all.

You don't have to switch.
When you start composing a new message and select a recipient, iPhone will detect if the recipient has an iDevice and/or has enabled iMessage, it will automatically treat the message as iMessage.

If the recipient name and compose message background is blue, it is an iMessage where as if the recipient name and text background is green, it is a SMS

Turn it on and leave it, it takes care of everything automatically. I have never shut mine off since the day I turned it on.

If you have no data plan, this can be maddening