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If you do.not mind changing cards once a year, just sign up for the card with the most lucrative enrollment bonus each year. Currently, you can earn 30,000 bonus AA Aadvantage Miles by taking the associated Citibank MC fee free for the first year, and charging $10,000 to the card in the first year. The bonus miles are in addition to the regular miles you earn from charging purchases to the card, so $10,000 in spend will yield 40,000 miles. After the first year (or after you charge $10,000) switch to another card with a substantial sign-up bonus and a fee waiver for the first year. United's Mileage Plus Visa is offering a first-year fee waiver and 21,000 bonus miles after charging a small amount to the card. You may also be able to get similar deals with both cards by taking out a business card as well. Find the credit-card offers at each airline's website.