After maining with Leona for a good amount of time, I have found several builds to be viable. However, this is one that I have begun to prefer, because it covers everything you need to be a super-tank.

I use my rune and mastery builds for early-game durability, then tailor my item build for the late game. There are a few rough spots if you get stuck before being able to buy Warmogs, but after that it should be fairly smooth, so long as you buy your items in a good pattern.

Your rune build is what you will be relying on through the early game. Flat armor seals, flat MR marks and flat HP quints will give you the survivability you need to deal with early ganks and aggressive lanes, particularly if you're soloing. MR-per-level glyphs give you some decent MR scaling into the mid-late game until you can get your Force of Nature.

I go a 0-30-0 mastery build because everything Leona needs is in Defense. There are a few tough choices, but for the most part it's fairly simple.

Take the summoner spell boost for heal/cleanse, if you're doing a different spell build you can probably exchange this for the second point in damage reduction.
>Armor and Magic Resist fill up your first tier, an easy choice.
>At tier 2, Health Regen per 5 should keep you in your lane for a long time, while HP per level scales very well.
>For tier 3, hit up Veteran Scars for 30 free HP, then put 1 point into damage reduction. Would like to spend more, but there are better options lower down.
>Tier 4, take everything. Tower armor reduction helps with pushes into the late game, esp. when you're tanking them for a Backdoor. Initiator helps you get into position in time to land clutch stuns and Z-blades. CDR per level helps round out your item build, as Frozen Heart is your only real CDR item.
>Take everything in tier 5 as well. 1.5% dmg reduction WILL save your life later on, and bonus gold from assists is nice, especially when you're going to mass assists, while getting few kills.
>Take the tier 6, because 3% total health and 10% disable reduction is hard to beat.

Open with Regrowth Pendant and a Health Potion. If you get into a big fight you can drain the potion instead of going B, and the pendant plus your masteries should keep you from having to go back unless you're almost dead.

Grab Philosopher's Stone on your first trip home to start cashing in on that sweet gold income, and buy Boots if you can. By your second trip back, you should have enough for Heart of Gold so you'll be sitting comfortably on gold income, especially if you're starting to get some kills and assists.

This is where you can tailor the build to fit your opponents. In general, it's good to get Warmog's Armor out and done quickly, so you can build up your HP and regen quickly to cover Leona's bases in HP. After that, check out what seems to be killing you most. If the other team has more AP, go for Force of Nature. If they have more DPS then go for Frozen Heart. These are your major MR and Armor items, and they cover a lot of other necessities as well, such as regen, move speed, CDR and mana.

If you get late enough into the game where you've gotten all these and still have money, start picking out some more items to round out your build. In general you'll want more HP, as you've reached the cap-off for what armor and MR does for you, but there are a few exceptions.
Locket of the Iron Solari is good because it builds right out of your Heart of Gold, gives a helpful regen aura to your team, and has an active shield that can really come in handy. It's not the most powerful item, but it's definitely within the realm of viability for Leona.
Sunfire Aegis is another good choice, as it bolsters your HP by a good amount, and gives you some decent AoE damage. A straightforward item, and a good choice for late game.
Guardian Angel can be a great item, despite your capped resistances, because the second life on such a durable tank can actually help you escape, or even get a kill or tower. For general usefulness, however, this is rated a bit lower than other items.
Aegis of the Legion is a great aura item, and is fairly inexpensive. You can potentially throw this in earlier in the build for its low cost and effective aura, but it's not quite as necessary because, while its effective values for team fights are high, your direct benefits are fairly limited.
Banshee's Veil is great if they have dangerous burst-damage mages or CC, and helps out your HP and MR at the same time. You don't really need that much Mana when you have Frozen Heart already, and its HP is fairly low compared to other items, but it's still a valid choice if you're up against an AP-heavy team.

Lastly, there's the discussion about Atma's Impaler being viable, turning your tank into an Atmog's killing machine. If you really need more damage, Atma's is a very good item for its cost, but a lot of times you're better off going pure tank, as in my experience I'm running around protecting allies and using direct CC more than I'm standing on top of one target and hacking it down.

No matter what you're item build, you're going to want to follow the same skill sequence, with only a few differences.

At level 1, get Shield of Daybreak. Period. This is your best skill because of the long stun duration, plus its cooldown decreases with level. Max it by level 9, every game.
At level 2, you have your first choice. Get Eclipse for general survivability and defensive play, or get Zenith Blade if you have an opportunity to jump in for a kill.
Level 3 is the last real "choice" as far as skills go; Either get Eclipse or Zenith Blade to have all 3 skills available, or keep leveling up Shield of Daybreak for a lower cooldown.
Past that, just put 1 point into the missing skill or Shield of Daybreak at level 4, depending on what you did at level 3. Put a point into Solar Flare whenever you can, max Shield of Daybreak by level 9, Eclipse by level 13, and Zenith Blade is lowest priority.

I go for Heal + Cleanse for summoner spells for a few reasons, but mostly because it just helps out your general tankiness, and helps you survive ganks throughout the game. Plus, as they're both increased by your level 1 summoner boost mastery, you get their full benefits.

This Leona build is great as a pure-tank. Runes help you float through Leona's normally shaky early-mid game, and your items should boost your stats to phenomenal levels in the end game. You should hardly have to recall, unless you're almost dead or have enough gold for a major item purchase.