1.
Take your time when learning how to enter text on the iPhone 3GS virtual keyboard.  Remember, speed is not the objective but rather accuracy and confidence and with confidence will come speed. 

2.
In many iPhone applications, you may enter text either in Portrait or Landscape orientation. 

NOTE 1:
Portrait orientation is defined as when iPhone is held so that its Home Button is located beneath the Display Screen.  Landscape orientation is defined as when iPhone is held so that its Home Button is located to either the right or left of the Display Screen.

2.
Although counter-intuitive, use a very light touch when exploring the virtual keyboard.  You will discover that doing so will greatly reduce overall tactile fatigue and greatly improve character selection accuracy. 

3.
I strongly suggest that when entering text, do not use the Double-Tap technique unless you find it difficult to locate a particular letter.  Instead, first, locate the character you wish to enter with one finger and then, with that finger resting on the selected character, simply Tap the screen with a second finger.  The selected character will be entered into the text field.

NOTE 2:
When I say Double-Tap only hard to find letters, I am referring to letters to which you may have difficulty in distinguishing based on the sound of the letter as pronounced by VoiceOver.  For example, in my case, I find it almost impossible to distinguish between the sounds of the letters M and N. 

NOTE 3:
At the time of this post, it is not possible to have VoiceOver announce characters phonetically.

Consequently, to confirm that I have the correct letter selected, I will slide my finger to a neighboring letter and then back to the intended letter, adjusting my selection if necessary.  Next, to make sure that I do not inadvertently, slide my finger over to a similarly sounding character, I completely remove my “selection finger” and Double-Tap the Display Screen in order to confidently enter the correct letter. 

4.
It is possible to either on purpose or inadvertently, position your finger so that it touches multiple letters that are adjacent to each other.  Of course, this will depend upon the size of your finger and the orientation of iPhone 3GS. 

If you do find that you are positioned between two letters, one being the intended character you wish to enter, instead of sliding your finger in such a way as to more correctly cover the character, simply tilt/roll your finger a little bit more so that VoiceOver says the name of the intended letter and in that instant, quickly Tap the Display screen with a second finger in order to enter the character in the text field.   After which, you may tilt/roll your finger in a different direction to select yet another adjacent character.  This technique will greatly increase your text editing speed. 

Good Luck,

Mark