Get in the habit of generating a new trial balance every time you record a journal entry (or entries) to double check your work. Did you need to debit or credit underbillings? Don't assume you did it right until you check!
I can't tell you how many times I used the wrong signs the first time I recorded an entry. Just like everything else, it's best to identify and address errors early on. Always check the trial balance against your work-paper to make sure the balances match in the end!
Remember, the trial balance is one of the most important financial reports you have! So make sure it's accurate!