I'd send one now and then one every 3-4 weeks, try to time one right around when they will make decisions.
What I like to do for each letter is expand on something in my application. A good start with why you want to go to that school. Or, focus on X quality of yours and why it makes you a good candidate. Update them on anything new you've gotten/done since your last application. Also, feel free to send in one extra LOR if you can scrounge one up. And of course, updated transcripts. The letters should be around 1 page long.