My current rule is that you roll all your hit die, and maximize your lowest die. So if you're a 4th level cleric, you roll 4d8. You roll a 1, 3, 5, 8. You maximize the 1, so your new HP total is 3+5+8+8 = 24 (+Con modifiers).
The other part of the rule is that you reroll your hit points every long rest; if your new roll is higher than your previous total, your new hit point total is the new roll. That rule sticks even across levels. I had a PC who rolled extremely well at 9th level, so well that their HP total didn't budge until 12th level.