"There's plenty of fish in the sea." Aren't you sick of hearing that one? Everybody says that as if it fixes everything. Yeah, sure... and what if the next fish is a stinker too? It's just like after a funeral when everybody says "Well, he's in Heaven now." George Carlin said that he would personally come down and kick the tail of anyone who said that after he died.
You want the dos and donts of getting your ex back, and here's the best piece of news: believe it or not, your ex is probably thinking the same thing. A relationship is like an investment. You don't want to sink all those costs into it just to have to walk away empty-handed.
Don't dwell on "What happens if I fail?" Instead, first resign yourself to the idea that you, all by yourself, have to be happy with yourself. Now, as a whole person, seek someone to include in your life. If you're in a panic without the other person, if you feel like your life is over without them, then you're being too clingy and dependent. This is a sign that you have serious personal problems to work through first, before you try being with anybody else.
Don't come back angry. Putting the other person on the emotional defensive is the last thing you want to do. And if you're still angry, it goes to show that you haven't had nearly enough distance yet.
Don't post ultimatums. You're not negotiating with a SWAT team, you're trying to strike a relationship with a former partner. Instead of "here's what you must do," it's "what should we do?"
Do be ready to fail. What, one more day? Everybody fails some days, and every relationship goes through twists and turns. If it's worth having, then it's worth working for.
Do take the full responsibility for anything that's your fault, and be completely open and honest about what has happened. You'd be surprised how many transgressions you can get away with if you just fess up to them! But for most partners, their number-one complaint is that "It wasn't the cheating (or drinking or whatever), it was the lying and breaking of trust."
Do be ready to communicate, communicate, and communicate some more. In the dos and donts of getting your ex back, this is the most important. Remember that this also means really listening to your partner, not just talking.