Haven't played flute in a year, any tips/techniques?

I used to play the flute, but I moved schools and I haven't touched my flute in a year. I decided on joining band again this year but I seriously fail at the flute now. I'm going to be on vacation so I'm missing 7 marching band practices in the summer...great. :-/ any tips or techniques on how I can improve my flute skills? Now I sound all airy and flat and sharp all over the place. I have tried longtones but I can't go past any note above Concert Bb (the middle one) without sounding like my flute is dying. Thanks for any help.