I can add couple more things:

1) Try to avoid using lots of symbols. For example: "***** BIGGEST THING! *****".

2) Don't make the subject line too long, because you don't know what email program your reader is using, and he might not see the whole subject line, so it might be turned just into "...". Example: "This is the greatest things you'll ever exp...". Try to keep your subject line 5-8 words (5 to 7 even).

That's what I can say from my own experience