What we translate as "Very truly..." is a tranliteration from Hebrew to Greek of the word (further transliterated into English) "amen." We usually thing of it as a nice ending to prayer. But in fact, it is what the linguists call an assertive particle. It would be more approriately translated as "Let it be so!"...a declaration of assent to whatever immediately preceeds or follows it.

So...enough linguistics. Why does Jesus say this all the time? (And indeed, he does: 31 times in Matthew, 13 times in Mark, 6 times in Luke and a few more in John). It's not that we don't trust Jesus to be telling us the truth. It's more like a textual clue to pay attention to what Jesus has said. In other words, "This is important, Bub. Listen up!"

I'm trying to think of a modern cognate for it, and am not sure there really is one. As close as I can come is when we might say "Listen to this." or "Pay attention; this is going to be on the test."

Hope that helps answer your question!

PD