So far as I'm aware, there's no better officialish spell than the 4th-level Sor/Wiz spell cynosure [div] (Dragon #308 77) for improving the accuracy of a plane shift effect, and there's no better officialish readily available feat than the spelltouched feat Accurate Jaunt (Unearthed Arcana 92) for improving the accuracy of a plane shift effect. I've a document that I think compiles all the Wizards of the Coast spells from the Dungeons and Dragons, Third Edition corpus that's dnd-3.5e inclusive and another that similarly compiles all the feats. Those spells and feats are largely it for doing what you want to do.
There are likely other magic items than those below that may be helpful in improving a plane shift effect's accuracy, and I'll add those to this answer when I locate them, but I'm not immediately aware of any, and the research I have done has yielded no better results than those below. (I mean, seriously, I'm assuming planar ring gates (Planar Handbook 83) (200,000 gp; 20 lbs. ea.) and the rope of crossing (PlH 83-4) (90,000 gp; 10 lbs.), while interesting, just are not what you're looking for, nor is using an effect like the 8th-level Sor/Wiz spell greater plane shift [conj] (Spell Compendium 159), no matter if such an effect is created through an attuned gem, rune, scroll, staff, skull talisman, or spellworm.) The same goes for classes: I know of no class that improves a plane shift effect's accuracy. (But if I find one, I'll add it!)
Outliers and also-rans
There's the poorly-named epic feat Dark Transient (Epic Insights Web column “Epic Warlock Feats”) that allows an epic warlock to
use plane shift with perfect accuracy (self and personal equipment only). You can also transport other creatures, but you then suffer the inaccuracies normally incurred when using this spell. Activating this ability requires a standard action.
Obviously, taking the feat requires that the creature be an epic level warlock or that the DM approves a very generous reading of the slightly controversial term bonus feat (and most DM's don't).
- A creature that takes the feat Touchstone (Sandstorm 53) et al. can establish a link with the Great Orrery (Planar Handbook 178-9) on Mechanus. When the creature gains the touchstone site's higher-order ability, he can 3/recharge use an effect like the spell plane shift except that he's only off by 2d10+5 miles instead of 5d% miles.
Although expensive, an amulet of the planes (DMG 247) (120,000 gp; 0 lbs.) is 100% accurate with a successful Intelligence check (DC 15). Far less expensive is the lesser amulet of the planes (Dungeon #107 69) (24,000 gp; 0 lbs.), which is identical to the original except it functions but 1/day. Note that only the DM stops an not-distracted, not-threatened creature from taking 10 on ability checks.
As an aside, creatures employing plane shift effects to reach the Seven Mounting Heavens of Celestia "always find themselves in the surf of an ocean surrounding Mount Celestia" on the plane's first layer, Lunia (Manual of the Planes 133-4). So at least where a traveler shows up on that plane is somewhat predictable. Other planes apparently aren't nearly so accommodating.