Focus on the symbols
The environment items may hold goodies for you so watch for the symbols, typically of coins or bullets (a glossy star-like bling for a split second). Shoot the item that they symbols appeared over and you will get either the coin or the bullets when you see them. Due to the fact that a coin is not worth the lost bullet-- you will see that you will seldom have enough of those anyhow, I would suggest to do it mainly for bullets.