The 2nd best tool I have for avoiding a nasty ding is my Garret Propointer. The best tool i have is the detector itself. Once you locate a target, learning how to size it up and pinpoint its position in relation to the coil is a must. Coins in parks are generally the easiest to practice with. Odd shaped relics and coins in nail and iron infested areas are more of a challenge. Once you find a target, dont skimp on time. Take your time to circle the target like a buzzard. Pinpoint from different angles of approach. Doing so will narrow it down dramatically. Always dig a little deeper than you believe the target to be. Once you have cut your plug or flap, the Garret PP will do the rest. If the item is still in the hole, the PP will tell you. To pinpoint its precise location, turn the PP off, hold it in the hole near the item and turn it back on. This will reduce its sensitivity. The reduced sensitivity will help pinpoint the location exactly, showing you where it's safe to dig. If the item is stuck in the side of the hole, you will now be able to push the digger in right above it and pop it out without damage.
Remember, the key is always to practice! I find that coin shooting in popular parks is the best practice. Treat each coin as if it were a gold coin and you will be (mostly) ding free in no time! Also, there are composite trowels available that can help with retrieval once a plug or flap has been cut.
Happy Hunting