Boys often leave the "be" off of because when they talk. What is the best way to write that, cuz or 'cause?

I believe 'cause is the preferred spelling, the word "cuz" is also acceptable, but can be easily confused in a dialogue with the shortened word "cousin." (For an EXCELLENT example of using "cuz" for "cousin" watch "The Godfather III").

On the other hand, somtimes, the PRONUNCIATION is clearly best expressed as "cuz" and not 'cause, in some cases, I think "cuz" could be the best way to write it, but for the most part, I would use 'cause.

I go with "'cause" with a ' on the front of it.

"We'll do it 'cause I said so." - for example.

In English English and possibly American English use 'cos.

As in:

"I had his balls cut off 'cos I wanted stop him fucking other women," she said.

It depends partly on who your audience is. It can also depend a little bit on where your story takes place. For instance if it takes place in Hawaii you might want to go with cuz which can also be taken to mean cousin depending on what the context is. So it just depends mostly on what you want to convey.