Normally when doing walking meditation I use buddho, alternating each syllable with every step and focusing on the feeling in the feet, but another technique I've found useful is practicing extended amounts of metta meditation doing walking meditation. For this I normally use 9 or 10 categories of beings to direct metta towards, and actually coordinate the metta reflections with the length of the path. For example, I start out with "May I be free of danger" as I'm walking, get to the end of the path then stop and turn and begin walking again, "May I be free of sorrow", get to end of path, stop, turn, begin walking, "May I be free of pain", get to end of path, stop, turn, begin walking, "May I be at ease". I repeat this for about four minutes then move on to the next reflection, teachers, then parents, then relatives who I'm close to, then friends, neutral beings, beings who have done me harm, then all beings.
Usually I spend around 40 minutes at it, and I find this practice very useful because normally I get very restless practicing even five minutes of metta sitting down, but coupled with the walking it holds my interest fairly easily. Anyway, it's a little unconventional but I thought it was worth sharing.
"The heart of the path is quite easy. There’s no need to explain anything at length. Let go of love and hate and let things be. That’s all that I do in my own practice." - Ajahn Chah