make sure that your parents and siblings are asleep, i mean DOUBLE check!
and dont come back to your house unless you are done for the night, if you forget something it would cause too much extra commotion to go back in and get it (my friend went back to her house once after sneaking out to go to the bathroom and she got caught).
start sleeping with your door closed (if you dont already), but if you normally sleep with it open, dont close it for that one night only, itll be too suspicious.
do you know if your parents check on you at night? (i know, its lame, but some parents do). to see if they do, close your door about 3/4 of the way and put something small behind in, like a bottle of nail polish. (but not something large like a t-shirt as it may get stuck in the door). check in the morning to see if the object was pushed back by the door, if it was, then your parents (or maybe someone else) opened your door during the night to "check on you" or whatever. having your parents check on you in the night makes sneaking out a lot harder!
going through your window is a bit dramatic, i think. just go out the front or back door. (check which is quieter during the day, before you go anywhere). if its too squeaky, spray it with WD40. (found at any hardware store, or maybe even at a rite aid or a CVS). this will make the hinges on the door open easier and quieter.
and dont go out through the garage. i know DUHH but one of my idiot friends opened the garage once trying to sneak out and it woke up her entire family. wow.
remember to pack these items: house keys (you have to lock the door before you leave for your family's safety), a cell phone (in case of calls from worried parents. and yes, answer your phone if they call you! youd dont want to be chased down by helicopters and searchlights!), and pepper spray/maize, and/or a pocket knife. (just to be safe!)
if youre really scared of getting caught, take your pajamas in a bag with you and change back into them before you get back in your house. your parents will more likely believe your backup story if youre in your pajamas and not your party clothes. and when returning home, dont be drunk.
okay if you do get caught, dont act nervous, dont fidget, and dont babble. have a plan ahead of time. if they asked where you went or whatever, create a story thats not too crazy and wont get you in as much trouble as the real story would, if any. try:"i just needed to clear my mind" (if recognized as emotional), "i thought i heard (insert dog/cat name here) scratching at the door outside" (if you have a dog/cat), "i was checking out the night vision on my camera", (it might work if you are an aspiring photographer).
so, have fun sneaking out! enjoy it and please email me and tell me how it goes! if you have any more questions about sneaking out, just ask!