Android's Voice-to-Text feature is surprisingly accurate, but even if you haven't picked it up as your go-to input method, you can still use it when you want to make sure those difficult-to-spell words are correct.
Lifehacker reader waterskier9 explains:
Quick tip for hard to spell words if you are using a smartphone: When typing an SMS message I want to spell everything correctly, so when I need to spell a word and I'm not sure how to spell it, I just use the voice dictation feature for that one word. Works great on my Droid Incredible...especially when I want to spell 'Chauffeur'.
Since your keyboard likely has a button in the corner for invoking the feature, it takes very little time to use this technique. That said, for straightforward messages, if you haven't started using the voice-to-text feature... you need to try it. It gets nearly everything correct. Of course, it still doesn't catch a lot of your more conversational nuances or oddly spelt exclamations ("Duuuuude, wut!"), so on those occasions you're still using the regular keyboard, this tip can come in handy.