How to use Chapel card to its fullest potential?
Chapel can be effectively used mid and late game, but that requires card draw or action branching or both. 

The trick to chapel is to trash lots of cards with it early and not rely on it late game (turn 10 bad mid-late game cards into 1 dead card early). This means trashing your entire starting deck as soon as possible. Do not wait until you have plenty of silver + gold to trash coppers. You should consider trashing coppers immediately and buy 2 silvers (or something to get you to at least $3) when not trashing 4 cards. 

Ideally using the above strategy you will end up with a deck &lt;= 5 cards that contains 1 chapel and 2 silvers (or 2 cards that will make your hand >=$3 each turn). From here you have slim deck that can be quickly transformed to use whatever actions you want from the supply. The next couple of buys you make will see use almost immediately, while your opponents will each have a couple of hands before new cards get to see use.

If you hold off on trashing until later in the game, you are going to have to deal with the following situations:
1. drawing chapel when you do not draw anything you want to trash. 
2. drawing chapel when you have a different action in your hand you want to play.