Know what's on your credit report
Most people have not idea what is on thier credit report. They also have not idea what their credit scores are. This is amazing to me, since the way to financial freedom is to manage your credit health. Otherwise you are probably paying too much in the way of terms and rates. Get a copy of your credit report with credit scores regularly to mange your scores.
Pay all your bills on-time
If you think paying a bill late every once in a while is ok, you might want to rethink that logic. A single late payment on any obligation will lower your credit score between 100 and 150 points. So if you had a credit score of 750, now it's a 600 score. If you are having financial problems and think you might be late, call the creditor to make arrangements. Make sure once you work out something, that you get something in writing stating they will not report any payments late with the arrangement. So the idea is to not be late on anything. Lenders don't like to see you late on anything, because this looks like you are having financial problems. This ultimately means you are not ready to buy in a underwriters eyes.
Save your money
When applying for a mortgage, having savings is a big plus. This shows you have stability and the ability to save money for emergencies. Most loans are run through a automated underwriting engine, and with savings in the bank could mean the difference between a approval or a denial. A good goal to have is 6 months worth of mortgage payment in the bank. If you are buying a house around $200,000 six months payment would be $12,000 in savings.
Pay your Rent on-time, and don't break a lease.
Being able to pay rent for at least 12 months on-time is a plus. It looks better to pay rent and all your utilities on time for 12 to 24 months. This shows a underwriter you are able to handle responsibility. Lease agreements are one of those contract deals that if you break it and don't pay the fee to break the lease it can show up on your credit report, plus the apartment complex typically will not give a good verification for rent due to the lease being broken.