Drink a lot of water, eat enough food and extra protein (maybe get muscle milk light??) an SLEEP! Well, you probably won't have a choice on the sleep
DH and I did it together before the wedding and we just started it again 6 days ago (we ordered Insanity from BeachBody and started it but hated it compared to p90 so we returned it and decided to start p90 over!)....
I do a lot of modifications (ie - pushups from my knees when I get tired, steps instead of jumps at the end of plyo, etc)......don't be afraid or annoyed about doing modifications - by the end you will totally build up!
Bottom line advice - STICK WITH IT! You will see awesome results! Have fun and good luck! I always love talking about p90 so give a shout whenever if you'd like!