Many mobile devices allow use of an optional, external keyboard (usually bluetooth). That is probably 'best' for extended typing - though you will need a surface to put the keyboard on, and some way to stow it for travel (it won't typically fit in a pocket or purse).
Another way is speech-to-text. I have Dragon Dictation on my iPod Touch and it works reasonably well.
Finally don't underestimate your ability to adapt to tiny keyboards or touch keyboards. It will never be as fast as a full size keyboard but especially with an auto-complete feature it will get much faster over time.
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