I get the idea of double entry, but I get confused about what to debit and what to credit sometimes. Journal entry quizzes kicked my ass.
Pick a simple example you can always remember and think about the logic of it in other situations to guide your line of thinking. My preferred one is (assuming a cash basis of accounting):
Sale to customer:
Debit cash in the amount of X to reflect the increase to your cash balance.
Credit sales in the amount of X to reflect the increase in sales for the period covered.
Let's say that now I'm presented with a journal entry to reflect the sale of new stock in the company.
I'm receiving cash, so I know I'm going to be debiting cash.
What am I going to credit? Even though I sold stock, I wasn't selling product. I was selling equity. Equity... wait a minute, that's it. I credit an equity account.
(These are obviously simple situations, but I hope the point gets across.)