Here’s one we’ve run before, but it bears repeating (based on watching several players who weren’t following our advice recently…): Always tune a stringed instrument, such as a guitar or bass, UP to pitch — start with the string tuned below your target pitch, then bring it up to pitch. This prevents slack in the string that will later cause the tuning to go flat. If a string is tuned sharp (above pitch) bring it down below pitch, then back up to the correct tuned pitch.
Tuning in this fashion will result in a much more stable instrument, tuning-wise.