I very nearly passed 21 days on polyphasic sleep last summer before giving it up because it was too incongruent with my family's monophasic sleep schedule. What I did that helped:
-Make a chart with checkboxes for every nap for the first 21 days (if you can make it past the first 21 days, it's a habit). Check off your naps religiously, and mark down exactly when and if you oversleep. Oversleeping is not nearly as painful to think about if you have an otherwise good record to look back on. Also, thinking only about the first 21 days and knowing exactly where you are at all times can help to keep you motivated.
-Sometimes when I went to bed in the afternoon, I would have a bunch of thoughts in the mind that would make it harder to sleep if I was feeling energetic that day, or I wouldn't feeling like napping then. I would lay down, tell myself once to "think of nothing" and clear my mind. Even though my thoughts would drift afterward, this method seemed to lower the overall stress level of my thoughts, making it much easier to sleep on those occasions so I didn't have trouble later.
-Tell as few people as possible in the first 21 days. I told no one until Day 7, and I think I should've waited longer. As soon as people hear you're going polyphasic, they'll start telling you it's bad, which is no good for your brain (on Day 7 mine was still pretty sleep-deprived, and I needed as little criticism as possible). If I was going to do it again, I'd wait until Day 21, when I've made it a complete habit, to tell others (besides, of course, the supportive and encouraging people on this forum).
I hope these suggestions help, and either way I hope your polyphasic sleep experience goes well! I've thought about going polyphasic again several times--it's just such a cool way to sleep!