If your charms ever backfire, try to dodge it, and do NOT use that wand again. Go to the nearest wand store (Mr. Ollivander's), and either

a) Ask Mr. Ollivander to look at your wand and try to fix it, or

b) Buy a new wand.

Some people, whose wands sent backfiring charms out, ignored it when it happened. The following is about a person who just carried on using the wand.

An example is Alina Gosenwaber. The first time her charm backfired on her was when she was trying to Banish (using the Banishing Charm) some pesky gnomes. I t backfired and she went flying backward and hit her head so hard on her brick wall, that she had to take 2 whole doses of Skull-Mending Potion!

The next time she used it, (which was pretty foolish, if you ask me, that she dared to use that wand) she was scuba-diving in the Caribbean, when she ran into a grindylow. "Relashio!" she yelled. But it backfired and she got burns all over her face. So she sent up sparks that spelled out M.O.M * (basically S.O.S.). The Ministry came, and she learned she must get a new wand.

I hope you learned what to do when you have a wand that backfires, so if this ever happens to you, just think what happened to Alina Gosenwaber.