Table alignment and sizing problems

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I seem to have a lot of problems with poor table alignment and column widths.

Quite often, the table headers don't lineup with the rows beneath. It appears that one is aligned to the left, whereas the other is aligned to the center.

Also, in a number of places, the column widths aren't wide enough, so it wraps onto the next line, looking really tacky. Here's a picture of the Account History page at /store/index.php?main_page=account.

How can I fix these problems?