Older account age = higher score

So opening new accounts lowers the average age of your accounts, and lower your score. Some scores give extra weight to your oldest and newest account, so opening a new account drops that one metric down to zero. Closing an old account can also have a negative impact. Your accounts get older every month, so this ding will gradually disappear.

Seeking new credit = lower score

If you had two friends, and both of them asked to borrow some money, would you rather lend money to the friend who never asks anyone to borrow money, or the guy who is constantly borrowing money. Even though the guy who is constantly borrowing money has more experience borrowing and repaying money, most of us would prefer to lend to the guy who seems less eager to borrow. This ding to your score will also gradually disappear.

Higher ratio of term loans to credit cards = higher score

Taking out a mortgage might actually increase your score, but opening a new credit card could decrease your score, especially if the balance of that card represents a high percentage of your total credit. This ding will not disappear on its own, but will fix itself if you take out a mortgage or car loan in the future.

Higher total credit available = higher score

For people with extensive credit histories and long account ages, applying for and opening a new credit card can actually increase their score. If you have a short credit history and/or don't have a lot of accounts open, then the benefits of opening a new credit line will take some time to eclipse the other factors mentioned.

Lower percent credit utilized = higher score

Going from one CC with a $8,000 balance and a limit of $10k to two CCs, each with $4k balances and limits of $10k would tend to increase your score. Under 1-30% utilitization is generally the sweet spot. Some scores will ding you for 0% utilization (having credit cards and never using them).