This is one of the issues that inspired this part of my blog. I hate seeing simple mistakes made by intelligent people who just never learned or realized there is a "there," their," and a "they're" or a "your" and a "you're" or a "then" and a "than."
Here's how I distinguish the difference. I think a teacher of my own showed me this, but I don't remember who--thankless job, that teaching.
then = time
The "e" in both words helps me remember.
Sophie and Michael played on their tummies.
Then Sophie got tired.
Michael lasted longer than Sophie this time.
As you can see, "than" is used for comparisons.
You might think of the "greater than" and "less than" symbols (> and <) as sideways A's to help you remember to use the word with the "a" in it for that purpose.