Here's a good strategy when using my Galaxy Class ships:
About 50km from the enemy, fire a full spread of photon torpedoes, and then in-system warp straight at him, dropping out of warp about 10-15km behind him. Then fire a full aft spread. The two volleys of torpedoes should crush most shields when the impact at roughly the same time, all before they get a shot off on you. Plus, since the Galaxy has excellent all around phaser coverage, while most enemy ships have poor aft coverage, you will be in an advantageous position.