Trostani has consistently been one of my favorite commanders since she came out in RTR. Here's a rundown on the way I've built the deck:
Synergy's Voice - Trostani has two abilities. The first is triggered ability that cares about the toughness of creatures you play. The second lets you clone your best token. The first only wants to put a lot of toughness into play. It doesn't care about power, it doesn't whether it comes in the form a one big or lots of little. It takes no additional mana to use, and rewards you for doing the kinds of things you want to be doing anyway. Experience tells me that it's more powerful than it reads, more powerful than the latter ability. The second is only as good as the best token you control. It costs mana, it limited by tapping. It can do good work, but it requires a lot more from you up front. Unlike the first ability it doesn't work well with lots of little guys, it really wants you to play big (token) guys. Altogether, the cards that synergize best with Trostani are cards that make singularly big tough tokens.
Lifegain Lifegame - Just having Trostani on board puts you ahead in wars of attrition. It's pretty common to hover around starting life when the rest of the table is down in the teens and twenties. You can take element of her card design and run with a little bit. Cards like Archangel of Thune, Well of Lost Dreams, and Rhox Faithmender are the easy picks that amplify your lifegain plan. You can also pay that extra life to Phyrexian Processor and earn it back right away. The real power of the lifegain element comes from the Serra Avatar. Serra Avatar is the best known of a subset of creature producing effects that make a total power/toughness of creatures in a way that relates to you life total. Ajani Goldmane makes a populatable token of Serra Avatar. Storm Herd trades the avatar for constellation of Pegasi. Ajani, Caller of the Pride goes wide with Cat Tokens with total p/t twice that of the avatar. Play em all.
Stay Awhile - Nacatl War-Pride is a great example a creature that synergizes greatly with both of Trostani's abilities. When it attacks, it has the potential to put a lot of toughness worth of copies onto the battlefield. Trostani also lets the kittehs attack fearlessly, because she can populate one of the token copies, and the populated token will stick around when the originals get exiled. (Ive managed to attack with multiple copies accrued over turns and turns, its awesome.) This same populate works with a few other sweet cards that make temporary tokens. Seance lets make copies of anything that found its way to graveyard and Angelic Favor lets sneak in blocker to mess up someones combat math. Mimic Vat is practically a staple in EDH and really shines here when you add some permanency to its mimicry.
Assembling the Ranks - Tokens. Theres are lots and lots of tokens in magic but which ones are best with our tripartite leader? Big tough tokens. Beyond what already been discussed lets look at few more standouts. Mitotic Slime is the Nesting Doll of token producers. It resists removal and represents 12 toughness for 5 mana over its lifespan. Reach of Branches is a gift that keeps on giving. Its may not be the most powerful spell, but it plays into what the deck is doing so well and vanilla 2/5s are surprisingly good at blanking opposing threats. Armada Wurm is 10 toughness for six mana and brings 5/5 trample token to seed your army. Phyrexian Rebirth can get you out of tight spot and it leaves behind a swell target for populate. Entreat the Angels isnt as good as you might think, but its still good enough to earn its place, even when it only making one angel for 5 mana.
Filling Out - Honorable mentions include the aforementioned True Conviction, it turns all your midrangey creatures into game winning threats. Darien, King of Kjeldor with Trostani means that opponents cant profitably attack you. Frontline Medic leads to a lot of sick lines, letting you playing sweepers in the second main phase with impunity. Adarkar Valkyrie's ability to save and steal creatures is very powerful.
To WWGG and Beyond - When I pick out ramp for Trostani I personally look for cards that you cast Trostani reliably on turn three. Given her restrictive mana cost that means it need to do double duty as fixing. Aiming for turn three Trostani means being able to cast these spells on turn two. Green Sun's Zenith for Birds of Paradise, Farseek for Temple Garden, Nature's Lore for Temple Garden, and Search for Tomorrow suspended on T1 all fit the bill.
In Short - Lifegain midrange. Play creatures, gain life. Make tough tokens, populate tokens. Play creatures with p/t related to life total, double life total (or more). Make team indestructible, wrath, swing.