That depends on what the spell you're trying is. Follow the directions, if none are given, assume that you need no special preparation. I would always meditate before a spell, in any moon phase.
Moon phases are important to casting spells, The moon is at its highest energy peak when it is full (the moon will be full today at around 17.05 in the northern hemisphere)
A Full Moon is best for spells of increase,protection,divination and is also a good time to honoring the moon.
During a Wanning Moon spells work best for decreasing and letting go of things such as bad habits or banisnment and curse removal.
During a New Moon/Dark Moon.. some people consider this a time to rest from magic but it is also a good time to work with dark magic and connect to dark deities a New Moon is also good for binding and banishing spells.
During a Waxing Moon its best to cast spells for increase and attraction of desires such as prosperity, abundance and love spells.
Spells have a more successful outcome if you cast the spell during the proper moon phase for the spell you are using...so say you want as you said to stop your smoking habit your best phase would be during the Wanning Moon or if you wanted to increase your will power to help you stop smoking then your best phase would be during a Waxing Moon.
The Full and New Moon are when the moons energies are strongest that's why a lot of spell casting is prefered at these times although you can cast a spell during any moon phase it just comes down to arranging the the wording of a spell to get the best affect so for you to cast a spell tonight for helping with stopping your smoking habit you would want to cast a spell for you to increase your will power to help you quit.. if it was a New Moon then you would want to cast a spell to have the urge or desire to smoke to go away, so it really all comes down to picking the right words to use during each moon phase.
As for preparing before hand, its best to have everthing you need before hand all in the one place so once you've cast your circle(if you use one) you wouldn't have to break it to get something you've forgot, then as said before meditate on the outcome of what you want your spell to acheive and have a clear indication of what you're going to say and do for your spell as in.... will you say something for each time you light a candle or past a crystal through incense or a flame ect, it all depends on what you feel is right.
