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
Si tacuisses, philosophus manisses - If you had kept quiet, you would have remained a philosopher. (Boethius)