The first is, he that vseth not to wasshe well hys mouth euer when he hath eaten his meales, shal I haue alwayes yelowe teeth & a stinking breath. The seconde is, he that slepeth with his mouth close, hath likewise an euyll breath, and foule teeth.
The thirde is, that for to maintayne and kepe the teeth faire and white, and a sweete breath, when a man is layed is his hedde, and when he waketh in the mornyng, it is good to purge well the breaste and throte, spitting out all that is gathered together that nyghte: which also is good for the stomacke and heade. And hauing your teeth and breath warme take a lynnen cloth or the corner of the shete, and rubbe youre teeth well within and withoute, to take awaye the fumositie of the meate, and the yelownesse of the teeth gathered together in the night: for it is that, that maketh youre teeth yelowe, and gommes redde, and corrupteth your breath. This is a verie necessarye thing to be knowen, and oughte well to be obserued: It is also good to eate euery mornynge some graynes of Masticke."
These are actually good pieces of advice (mostly). It is a good idea to wash your mouth after every meal, and to wipe them clean; this author recommends a linen cloth (like your sheet!). I'm not sure about advice #2, whether sleeping with your mouth open or closed makes a difference.
Chewing mastic is pretty good advice too; mastic is a gum resin (like frankincense) that is still used in the Middle East for cleaning out your mouth, often in the form of chewing gum. You might be able to find it at a Middle Eastern grocery store.
Source: The secretes of the reuerende Maister Alexis of Piemount, 1558