Like conventions there are a wide variety of signals used through the play to indicate a preferred lead. Discard signals are covered on a separate page, this page explains some of the signals that can be used when following suit.
One of the most common signals used is to play a high card following partners lead to indicate a preference to continue that suit.
One of the main failings is that this can be a waste of a high card so a partnership should consider an upside-down signal. This is where a low card shows a preference and a high card suggests a switch to another suit.
The Cincinnati signal extands this by showing a suit preference. A high card which is ODD requests a switch to a higher ranking suit. A high card which is even requests a switch to a lower ranking suit.
There will in some cases be alternatives which have to be thought by the person on lead. Following suit with a high even card to a heart lead could suggest either a diamond or club, whereas an odd card can only mean a spade (in this example).
Another useful signal is used to convey the number of cards in a suit. This is an extension to the high-low doubleton which indicates 2 cards in the suit.
This extension suggests that any high-low shows an even number of cards in the suit whereas a low-high shows an odd number of cards.
With S876 follow suit with the six then the seven. With S8765 follow with the 6 then the 5.
An immediate issue can be seen as in a trump suit the person leading the suit will not know whether this is a doubleton or 4 or 6 etc. cards in the suit.
It can be particularly useful in NT to show length, where a ruff on the third round is not relevant.
There will be instances where there may be conflicts in signalling for a suit preference or showing length. These need to be considered when decideing which method to use.
If there was a 'right' way, everyone would use it !!!