There are many myths when it comes to your credit. I'm not a mortgage person, so I don't know everything that goes into what can be ignored on a report and what can't, but I've learned quite a bit over the years and here are some of the important things to remember.
If you have several credit cards, do NOT close your accounts, even if you're not using them. Keeping these accounts open show a longer length of credit history which increases your scores. Also, your credit score is based off what you're allowed to borrow verse what you are borrowing. If you have several cards you don't use, you still have that money available to you, so you're showing a lower rate of debt, and that is good!
If you have cards that have high balances, paying them completely off does NOT help you. Your credit score is also based off consistancy. If you have high balances one day and no balances the next, it could actually hurt you because there is no proof that you won't go charge your cards up again. The best thing to do to pay them off and increase your score, is to pay larger amounts then the minimums, but spread it out over a few months.
Also, do not be afraid to shop your loan. Many people think that their score suffers each time their credit is pulled, but this is only true in extreme circumstances. The rule of thumb is that if you're shopping within a 10-14 day period and have your credit pulled several times, you're ok. If you keep pulling your credit over and over again over weeks and months, then, yes, your score will be effected negatively.
I hope these tips have helped. There are still several things to consider when trying to improve your credit, but these are the most important.