For those going the potted route, elemental sulfur is very effective, but you'll want to be judicious with its application. It's very easy to be impatient and use too much as it takes some time to break down and take effect, and can sometimes get caked in and create super acid zones depending on the soil you're using (typically soil that's very compacted and low draining). While it wasn't on trees, I got a little sulfur happy with my peppers last season and half of them died from the soil becoming too acidic for the roots to handle.
To keep pH levels lower, I recommend a combination of acid fertilizer (the kind used for blueberries and other acid loving plants, which often includes just the right amount of sulfur) and bi-annual use of FoxFarm's Sledgehammer (mid-fall and early spring) which washes away soil salts. Salts from high pH hard water are unavoidable in my area, and this helps tremendously to counteract its effects in addition to loosing hardened up, hydrophobic soil. It also helps "cleanse" our native soil whenever I'm inclined to incorporate some with other soil mediums.
Also try to keep a decent soil worm colony in your larger pots. They help keep the soil aerated, and thusly drain better and wash away salts easier, thus slowing high pH creep.