An Addendum to last TTT, for another angle at Internal Conflict that I didn't cover--but boy, do I wish I'd gone into that particular specific--take a look at Julie Cohen's Blog. She discusses IC as a state of mind--self-esteem, etc--that is not brought upon by singular incidents. I still hold that ultimately, IC is an inability to trust, either in one-self or in others, but Julie has great insight here and it's worth your time to check it out.
And now, onto External Conflict.
If Internal Conflict is the battlefield, EC is most assuredly the War Room of your particular book. In military terms, the War Room is where people decide if you live or die, where decisions are made for the soldiers, either against them or for them to carry out. As a writer, the War Room is your tool to make those decisions that will effect the overall war your characters are having with themselves. Don't fool yourself, if your character is not at war morally, physically or emotionally with themselves and others in the course of your book, you're not writing a complete story and that's going to cost you.
The essence of EC is my very favorite catchphrase in writing. "Trample, Trample, Trample". Have you ever gone to a party and someone says something horribly crass or accidentally mentions the ex in front of the new wife? The afflicted will blanch, their mouth will tighten incrementally and they'll try to smile it off, but you know that they were effected. That, my friends, is trampling. In real life, you feel really bad when you do it. In romance, it's the stuff that awards are made from.
An External Conflict is a situation that puts your character in an untenable position. Every decision will have a cost that your character is in someway unwilling to pay. The trick to making EC work for your book is to offset the Internal Conflict of your characters with it and use it to poke, stab and punch their issues. EC is their problem, IC is their weakness. Your job as the writer is to make your characters as uncomfortable as you possibly can so that they will face their inner demons to achieve their goals. If you have a character who hates to fly, stuff them into a plane. If you have a character who hates dogs, drop them into a warehouse full of them on guard duty. Say it with me, "Trample, Trample, Trample".
Depending on what line you're writing for, EC has differing levels of importance. Sometimes, it's cliché; nothing more than a veiled prop to set your characters in action. Hero cannot acquire fortune unless he proves he is capable by living without money for a month. Or, heroine will earn grant for her dream business if she agrees to spy on her boss for his investors. A very light conflict that can be overcome by simply telling the truth--if you're willing to sacrifice the things you want for the value of love. This is a great romance staple and there's nothing wrong with it.
But if you want external conflict that is going to last you the length of a full size novel or anything over 55,000 words, you will need EC that has depth, has teeth and taps into your characters weaknesses. Find their weakest point and put them in a situation that attacks it. Then make it worse. Do not give them the ability to balk from it or escape it; but feel free to let them try. More importantly, don't ask the reader to believe that their inner heroism is what forces them into the choices they make. They won't buy it.
People are heroic when they need to be, usually by instinct. No one runs into a burning building just because it's on fire. They run in because they know someone in there can be saved. Or should be. That's the difference between being heroic and being stupid. Stupid characters deserve to burn; heroic ones deserve to come out alive. It's the one time in life when you get to be Justice. Choose wisely.