You could put your most used in FAVORITES, tap favorites tap contact. The easiest way is voice calling. With the phone asleep, tap and hold the HOME button til the phone app comes up, then just say, "CALL, John Smith, MOBILE", she will repeat what you said, if you don't say NO, then she will call John Smith's mobile phone. So far I haven't had to repeat myself but once, when I still had the gauze from having a tooth removed in my mouth. She got it the second time, even with the gauze.
Page 62 of the iOS 4.1 manual
Page 64 of the iOS 4.2 manual