Regrets about a bull market take two main forms:
1. Insufficient positions
2. Insufficient holding
To fix (1), you increase size and make more positions (including increasing the overall leverage of the portfolio, less liquid positions, and holding through "unreasonable" price + valuation movements). The cost - higher portfolio volatility, higher risk of ruin, and likely significant drawdowns from your portfolio's ATH. But if done well, you can earn quite a bit, though with a higher risk of depleting your portfolio.
To fix (2), you take profits on the way up and generally become more conservative the longer the trend continues (fewer illiquid bets, less leverage, more TP and trade management). The cost - lower growth potential and potentially "missing out" on the cycle of opportunities. If done right, you earn much less but preserve more.
Important: it is not possible to solve both tasks simultaneously.
Choose your poison.