I played it a few times, and found out that furiously pushing and pulling your arms back and forth for three minutes ensures victory.
I tried the whole blocking and dodging thing, and all it does is turn the match into a "Metapod vs. Metapod" (excuse my Pokemon slang) match.
I've bested many a Wii owner in Wii Boxing by doing just that. I'm actually undefeated... no lie.