We're somewhat informal about this. If somebody puts out a chip that's too large, somebody else at the table offers to change the chip from his own stack. Example: Player A wishes to call T150 and puts out two T100 chips. Another player will give the Player A four T25's in exchange for one of the T100's. The caller adjusts his bet and pulls the extra two T25's back into his/her stack. Alternatively, Player A asks somebody else to break a T100 before calling.
We discourage (make that really, really discourage) players from making change from the pot. We do not allow players to make change from other players' bets or blinds (e.g., by putting out two $100's and taking two T25's from the BB's blind, although every so often this is attempted). Our players understand the reason behind this: Less confusion over who has bet what.
No splashing the pot. Bets remain in front of the player until the betting round is over.
The dealer is responsible for making sure that the pot is right.
Disclaimer: These comments don't address all possible situations for betting and making change.
Praise: Our players are quite conscientious, and they are aware that observing good poker etiquette results in a better and fair game for everybody.
Observation: We play with lots of chips, which significantly reduces the need to make change.
