This might sound like a bit of a rant over something that should be common sense, but here it is anyway.
A mortgage preapproval is always based on a snapshot of your financial circumstances at a moment in time. Should there be any material changes between when the mortgage is preapproved and when you find a house, the amount of the preapproval (or even the approval itself) coiuld be in jeopardy.
Sounds simple, but here are basic things to not do if you are preapproved and shopping for a house:
Apply for additional credit (credit searches can impact your credit score)
Take on new loans, or take significant additional advances on your existing credlt cards or credit lines
Change jobs - new employment always comes with a probationary period
And perhaps most importantly, pay all of your existing bills on time.
In times of high real estate prices and tightening credit standards, you can never be too careful.......