Finding the right points on a waveform display can be difficult unless they all occur in the silence between lyrical phrases. Within a phrase, however, many of the words flow into one another. Immediately before a hard consonant is the best place to edit. The quick burst of air used to pronounce hard consonants, separates their associated word from the word before. This means that the edit will work even if there isn't a pause between the words.
Hard consonants are relatively easy to find on a waveform display because of their sharp peaks.
Another useful but more difficult technique is to place your edit points on fricatives. Because of the noise they produce, it's easy to hide an edit point within them. This means that you'll be editing within the word itself, so you should really only use this technique when absolutely necessary. It's difficult to match up audio segments this way, especially when the words are being sung. Even with the preventative recording measures I mentioned earlier, noise can still make its way into your tracks.