1: Sometimes patience is key
Going hard isn't always the answer, sometimes it's best to let your ADC farm it out against some lane match ups (Janna, Ali, Braum to name a few) Against those types of supports, you have to be careful when picking your fights because they can make hard engage very difficult. Against Braum and Ali, for example, I like to counter engage (Just don't stand to close to your ADC against Ali because he can pulverize you both)
Now, that being said...
2: When you have an advantage, press it. Hard.
Leona excels at snowballing a lane. With even the tiniest advantage, you can turn bot lane into a buffet for your ADC. She has enough CC to make bot lane a nightmare for her enemies if they do not respect it.
3: This should go without saying but.. VISION
I find that vision is double important on Leona because nothing can screw you over harder than going for an all in just have the enemy top/jungle/mid come out and ruin your day. Leona has no outs once she's in, so you always want to make sure you're not out manned in fights.
4: CDR IS YOUR FRIEND
Stack that shit, and stack it hard. At 40% your Q is on a 3 second CD, which is extremely powerful late game, as you can literally just stick to someone and near perma stun them with your Q
5: Sometimes it's better to sit back and peel threats than hard engage all the time.
Leona isn't the best peel support, that much is true. But she can still stop people in their tracks with her Q and ult, and sometimes (depending on the team comp of course) it's better to stick with your ADC and focus on stunning anything that threatens them and let the rest of your team do the engaging.