Magic words and material components are mostly just details for me, although sympathetic inflation with a balloon is a nice device, especially if it's being filled with helium. As for the effects, I'd like the spell to cause inflation on a condition the target has little or no control over. There's a great story on overflowingbra.com where the protagonist experiences breast expansion whenever she's embarrassed, which needless to say tends to compound itself.
Is this for a story you're writing?
If I made an inflation spell, it would require some form of contact- a kiss, maybe a touch- something to set it off. The any other contact of this type will either up the power of the spell or reverse it.
It would have a one word command "Bloat", "Balloon" stuff like that.