Most iPhone 3G users press and hold the punctuation key and then slide their finger over to the (.) to insert a period. But did you know that there is a quick way to insert a period, then a space?

When you get to the end of a sentence, simply tap the space bar twice with your finger. This will produce a period and then a space. You can then continue typing and repeat this with every sentence. It makes typing so much easier once you get used to it.

The same result can be accomplished by hitting the space bar with both of your thumbs at the same time.

Source: iPhone Alley - Use Two Thumbs To Type A Period