Si fallatis officium, quaestor infitias eat se quicquam scire de factis vestris - If you fail, the secretary will disavow all knowledge of your activities
Si fecisti nega! - If you did it, deny it (stonewall!)
Si finis bonus est, totum bonum erit - If the end is good, everything will be good (all's well that ends well)

Si fractum non sit, noli id reficere

If it ain't broke, don't fix it

Si hoc legere scis nimium eruditionis habes - Essentially it says, 'if you can read this, you're overeducated.'
Si hoc non legere potes tu asinus es - If you can't read this, you're an ass
Si minor plus est ergo nihil sunt omnia - If less is more, then nothing is everything
Si monumentum requiris circumspice - If you seek a monument, look around
Si post fata venit gloria non propero - If glory comes after death, I'm not in a hurry (if one must die to be recognised, I can wait)
Si sapis, sis apis - If you are wise, be a bee