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Come on El Tigre, this is the most obvious one. Never use your own cellphone to call your mistress! Seriously, how hard is this concept? Your wife can check your cellphone bills, check your account status or even check your phone for weird calls. Just buy a new pre-paid phone and use that instead. You'll want to always keep that on vibrate, so your spouse doesn't question why there's a weird ringtone going off.
As Mark Wilson suggested, if you're going to be dumb enough to keep keep your cheat-pal on your phone, save her as "Mom". Or "Grandma". Or "Chiropractor".
