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1. Always make your payments on time and never miss any if you can avoid it. Make sure you don't miss consecutive payments which is often worse than missing a few stand-alone payments (separated by a few months).
2. Make sure you have a few accounts with access to credit, but avoid signing up for too many (or signing up for several in a short amount of time). I'm not sure what too many is, but it is probably safe to say that double digits is pretty high (if you wake up one day with 10 or more cards).
3. Make sure your credit lines report your credit limit. If you have a credit card but it doesn't report your credit limit, then it's hard for future lenders to gauge how much credit you have been trusted with in the past.
4. Establish a credit history early, at least a few years before you plan on applying for loans. Short payment histories are negative, since it doesn't provide very accurate or reliable information on your payment habits for lenders. Below, it says shorter than 3 years is way too short and shorter than 7 years is risky.
5. Avoid running a high balance on your lines of credit. Lenders will worry that you are living beyond your means. Running a low balance frequently is the best for your score; running no balance may be negative since it doesn't provide additional information on your repayment habits.
6. Avoid applying for credit before applying for credit. When you need a loan, make sure you don't apply for credit cards the prior year since lenders worry that you may be trying to take on too many new accounts at once.