An excellent rule for writing LTEs is one that my mother taught me on another subject 65 years ago: "Frank, take small bites."
(And, I'll admit, it's one I always have to remind myself of. My tendency is to say it all.)
A good LTE deals with one subject and deals with it fairly briefly. Take Mitt's first general-election ad, for example. He makes 3 commitments:
Okay the Keystone Pipeline,
Give his fellow millionaires an even greater tax break.
Now any one of those is an excellent subject for a LTE. You can even write three LTEs on the three subjects -- to three different papers. You should not write one LTE on all three subjects; if you don't own the paper, it won't get printed.
You could say:
1) Mitt ("Governor Romney" in a LTE) achieved one thing when he was governor of MA; now he's renouncing it.
2) Obama says that the Keystone Pipeline needs study to evaluate the consequences. Mitt doesn't care about consequences.
3a) Well, it didn't work when Reagan cut taxes on the rich to spur growth; it didn't work when Bush cut taxes on the rich to spur growth; Mitt tells us that we've got to keep trying it 'til it works.
3b) Romney pays less than a third of his official tax rate of 35%; he'll take care of that if he's elected. Then his official rate will be only 25%, and he'll pay nearly half of that.
But you should say only one of these.
(I'll caution you that I've only read about the ad in another diary on dKos. When you do write a LTE, try to base it on information that you get closer to the source than third hand.)