Based on limited observations, primers of 13-16 bases are more efficient than longer primers. As a general primer design rule (RCA, PCR, etc.), try to avoid more than 3 Gs or Cs in the last five bases to avoid mis-priming.

It is critical that the 3' end is protected from the strong exonuclease activity of phi29 by including 2-3 phosphorothioate bonds.