Before you ever begin casting a charm, you should answer the following questions (checklist).

1. Do you have a wand?

-An essential part to any spell, whether it's charms or not. Many wizards have absentmindedly forgot this and had unfortunate accidents. If you are searching for a high quality wand, Ollivander's Wand Shop is the best place to go- at least in Europe. And don't use anyone's wand but your one unless it is absolutely necessary. Your spells will not have the same effect.
 

2. Is your wand in good condition?

-This is again an important part to any spell. Make sure your wand is free of any debris that could hinder its performance. Also, if your wand is broken, promptly repair it with Spell-O-Tape. Do not forget Ron Weasley, whose broken wand caused many problems during his second year at Hogwarts.
 

3. Do you know (and can say) the correct pronunciation of the charm?

-This is extremely vital, especially when two different spells sound the same. For example, take the Jelly-Legs Jinxs and the Leg-Locker Curse. Both start with Locomotor. However, the Jelly-Legs Jinxs is Locomotor Flubb and the Leg-Locker Curse is Locomotor Mortis. Although the "F" and the "M" sound are different, both start with you lips fairly close together. One small error could damage legs permanently.
 

4. Is your aim correct?

-Make sure that you know who, or what, you are aiming at. If you are dueling and do not have time to think of a specific place, at least chose a general area.

5. Are you using the "Swish and Flick" technique?

-The "Swish and Flick" technique is widely accepted among wizards and makes any charm more fun. It was developed by renowned Professor Clinton Swish, a previous graduate of Hogwarts.
 

6. Is your spell legal?