Not selling first
Since you’re selling one house to buy another, you should plan to sell before you buy so you don’t feel pressured to accept a below-market offer just to meet a purchase deadline. If you've already sold your home, you can buy your next one, no strings attached. If you do get a tempting offer on your home but haven't made significant headway on finding your next home, you might want to put a contingency clause in the sale contract which gives you a reasonable time to find a home to buy.
If the market is slow and you find your home isn’t selling as quickly as you anticipated, you may want to consider renting the property and putting it on the market later—especially if you’re selling a smaller, starter home. You'll have to investigate the tax rules if you choose this option.
Better still, find a way to eliminate this situation altogether by getting your agent to guarantee the sale of your present home before you take possession of your next one. Programs like this ensure the home will be off your hands before it becomes an issue so you can make your move free of stress and worry.
Failing to get pre-approved for a mortgage
Pre-approval is a very simple process that many homeowners fail to take advantage of. While it doesn't cost or obligate you to anything, pre-approval gives you a significant advantage when buying a bigger house—or any home for that matter. It allows you to know exactly how much house you can afford when you put an offer on the home you want to purchase and you already have the green light from your lending institution.
With a pre-approved mortgage, your offer will often be viewed far more favorably by sellers—even if it's lower than another financing-contingent offer. So make sure to take this important step.