As a standard I always deploy 2 archer units to the left and right of the bridge, so that all 4 of them can control it. Spearmen behind them with a gap through which my cavalry can manouver.

Now, as a decoy i used to have peasants, but they're too slow, so i started using 1 unit of Ginetes (they can throw javelins) and depending on the AI troops positioning, sometimes all i do is to send them across the brigde and back and surely a enemy cavalry unit will follow them and be (half) decimated by my archers. Other times the Ginetes will have to throw some javelins first.

After repeting this once or twice, i can usually get one unit of Ginetes across the bridge without being attacked.
Then i use them to split the AI's army. As many as 200-300 men have folloew them across the fields.
Here is where i have mo make a careful read of the enemy army, cause it's time to cross that bridge
There are several options, but the important thing is, the next unit to cross the bridge, cannot be stopped before it gets to the other side, it has to leave enough space behind them so that the rest of my army can follow.

Hope that helps
Al Shama'ar