United Mileage Plus members can earn up to 100 percent bonus miles on all United and United Express flights through June 18, 2010. Fly one roundtrip and earn 25 percent bonus miles, fly two roundtrips and earn 50 percent bonus miles and three roundtrips to earn 100 percent bonus miles.

The bonus process starts over again with your fourth roundtrip and you do not earn bonus elite qualifying miles. Registration is required at http://www.united.com/offer/mpd330 — If you didn’t receive an email from United about this offer, you may not be eligible. According to the fine print, “Offer valid only for original recipients of email invitation to this offer.”