I also use two different verbal cues for trot and canter to make the command clear that I want a CANTER and not just more trot. I use a double-cluck for trot (single-cluck for more extension at the trot) and kiss for canter. Some people just use the words "trot" and "canter" to make the distinction too.
Of course, body language is a biggie for groundwork. Cheval's "throwing your energy" technique works wonders as well. As a heard animal, if your horse sees you (her alpha mare) suddenly leap into a "canter," she'll want to canter too.
If using a lunge whip, I also use it differently when asking for a trot and canter. For a trot, I'll keep it kind of low and just encourage the forward transition (along with verbal cues). For canter I'll kiss and lift the whip higher in the air with more energy.
Do you use any verbal cues to ask for the canter when undersaddle? If so, be sure you're using the same cues when on the lunge.