Tip #1: Decide If You're Part of an Organized Church
Tip #2: Decide If You Worship a God, or Follow an Idea
Tip #3: Ask How You Pray
Clerics are prepared casters, and as such they have to spend an hour at a certain time of day praying to their patron for spells. Most of us hear that, and we picture a cleric kneeling in quiet contemplation, sending their thoughts up to the divine to dip a little power out of their well.
That's all well and good for members of quiet, contemplative, pious faiths. But what about those who serve raucous, violent, or artful gods? Would they accept prayers from someone just sitting quietly in a field, muttering to themselves?
As mentioned in Improved Initiative's What Does Your Spell Preparation Look Like? post, you can get a lot of insight to your cleric when you ask what their ritual preparation actually is. What time of day do you pray, for instance? And what environment is preferred? What rituals do you perform, and do you need any special tools in order to perform them? Do you drip holy water and oil onto your blade, and draw a sanctified whetstone down its length while speaking the warrior's mantra? Do you make a series of beautiful charcoal sketches, then light them in a brazier so the smoke will reach your goddess in the clouds? Do you whisper your prayers to an insect, a fox, or a tree, trusting them to bring your words to the nature god who empowers you?
Don't be afraid to get creative with it!
Tip #4: Ask What Your Vows and Sins Are
Tip #5: Ask Who Your Cleric Is, Behind Their Faith
Faith is the defining feature of a cleric. Without it they have no patron, no magic, and they aren't markedly different from most normal people. But a cleric is more than who or what they pray to. Their faith is a defining feature of their life, but it should not be the only feature of it.
So ask who your character was before they found their faith. Ask how their faith has shaped them, and how it's altered their life goals. Ask what their temptations are, and how they deal with them. Ask who their friends are, who their family is, and if they had to leave anyone behind to answer the call of the divine. Do they ever miss that life? Do they have hobbies? Do they want to retire, and pass the mantle on to someone else in time? Or do they seek to die gloriously in their god's service, either to become a martyr, or to guarantee a place in the afterlife?
Despite their borrowed power, clerics should also be people. And when an actual person practices a faith, it can often have some unexpected results. Embrace those, and see where it leads you!