Euphoria is the mirror of panic and does more damage to good traders. Fear stops people trading; euphoria makes them trade more, larger, and outside their rules, precisely when conditions are most extended.
The markers are recognisable from the inside if you look for them: checking the account for pleasure, planning what the money will buy, telling people about the position, feeling that the usual limits are for other conditions. Each is a signal to reduce rather than to press.
Write the rule while flat. After a defined run of wins or a given percentage gain, size is capped and reviewed at the weekend. See house-money-effect and position-size-creep.
Related: house-money-effect, position-size-creep, market-cycle-emotions, hot-hand-fallacy