Serva me, servabo te - Save me and I will save you. (Petronius Arbiter)
Si Deus pro nobis quis contra nos - If God is with us who is against us
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