I find that using quotes make it look as though it is said aloud. That is always my assumption when I see a quotation mark.Ryne Billings said:It's a bit of a tricky situation.
I use single quotations and do it just like dialogue. Like this: 'I like to think like this,' he thought.
Some people do it like this: I like to think like this, he thought as he looked upon the horizon. It sure beats doing it another way.
And then some people just do it like this: I like to think like this.
There are other ways, but those are the three main ways that jump to mind. The third option is best used in a first-person point of view story, in my opinion.
I avoid using italics in thoughts for a single reason though. Italics emphasize something in their normal use, and I'm not trying to emphasize thoughts.
I agree with Eric C that if you are doing it well in a novel that is in close 3rd, you don't need italics or quotes for interior monologue. Italics are traditional for it but now are considered optional.