1 - High outstanding debts - keep balances low on credit cards and other revolving credit.
2 - Late payments - note that paying off collection accounts will not remove them from your report. These accounts will stay on for seven years unless you get a letter of deletion from the collector.
3 - New accounts - If you’ve been managing credit for a short time, don’t open too many new accounts too rapidly
Understanding how your score is determined and what can affect your score will hopefully help folks to act more responsibly, creditwise.